just purchased a relay, wondering what the typical switching speed was. what the max frequency i switch it at if i ever need to
If you have the manufacturer & part number you can search for the datasheet. You can probably switch a little faster than 10 times per second. I think small relays such as reed relays are faster than higher-power relays because bigger relays have more inertia.
do relays have any advantages over transistors?
Relay's only real advantage is that they're truly isolated, and don't care whether you're switching AC or DC. Sometimes it needs to be isolated - maybe you can't share grounds, or you want to switch mains power and a triac is too much work, or you're replacing a switch on a device that you don't understand, and can make no assumptions about what's behind the switch).
For switching DC loads , transistors (specifically mosfets - BJT's generally have worse performance as switches nowadays) beat the stuffing out of relays.
Oh - relays also have a limited number of contact closures in their rated lifetime. See the datasheet. Yet another reason fets are better.
Relays have their place. They can be designed to have much higher isolation between the circuit being switched and the control input than FETs (important for safety isolation), they are much more immune to transients and surges which would kill FETs, and they switch off (or on as required) cleanly on power failure. Relays also exist that can switch many thousands of volts and very high currents, the contactors used in mains power distribution being a type of relay. FETs are however much better for most common applications.
Russell
Typical relay switching speeds are around 10ms, but will be longer to switch off
with a simple free-wheel diode (as opposed to a snubber network). Reed relays
get down to a few ms IIRC.
Relays have a switching life, probably in the 10^6 area, which is normally a limit
to how rapidly you can switch them (and have reasonable expectation of
device lifetime).
When I was a kid, I had one of those Radio Shack branded "150-in-1" kits - it had a small relay. One of the circuits had it wired as a buzzer; it wasn't very loud - but it switched rather rapidly. That said, I wouldn't count on doing that in real life (knowing what I know now) without expecting the relay to fail rather quickly.
One way to actually see the switching speed of a relay is to power it through a break contact of the relay itself.
Leo..
I'd treat your relay as a "clonk" switch which can be clonked once a minute and won't have the very high lifetime claimed by the manufacturer unless your coil supplying circuit is relatively well-mannered. Pick a number, for example 10000 switchings, and that is how many times you can switch it in the lifetime of your project. It is possible to break a relay in a day by screeching it with 50Hz.
The other advantage of relays as well as those mentioned above is that they don't care about the polarity of what is being switched. With a fet, you have to be careful and sometimes might want an extra diode to stop it from being always-on to backwards voltage.
ad2049q:
Pick a number, for example 10000 switchings, and that is how many times you can switch it in the lifetime of your project.
10,000 cycles? That's a bit pessimistic isn't it?
Typical control relays have predicted contact life at rated power of anything from 50,000 to 1,000,000 operations. Longer if de-rated. Relays designed for critical applications such as railway signalling have a life of at least 10,000,000 operations.
Russell
russellz:
Relays designed for critical applications such as railway signalling have a life of at least 10,000,000 operations.
And they are just a little larger than an Arduino. 
CrossRoads:
Service Life, Mechanical, 2,000,000 operations min. (at 18,000 operations/hr)
G5T-1A DC5 Omron Electronics Inc-EMC Div | Relays | DigiKey
Ah, but the electrical (contact) life is only 30,000 at 5A 250 Vac.
Having said that I have only ever experienced one relay failure. That was controlling the heater on a top brand dishwasher. It failed and burnt part of the pcb about a month out of guarantee. It was a 5A relay switching an 8A load. I repaired the board and replaced the relay with a 15A rated one and so far it has lasted about ten years.
Russell
You need really wide traces and good signal separation to do 5A/250V on a PCB tho. I imagine for that the relay would be mounted closer to the load and not off at some central location.
CrossRoads:
You need really wide traces and good signal separation to do 5A/250V on a PCB tho. I imagine for that the relay would be mounted closer to the load and not off at some central location.
Yes, it should have been but it was on the control pcb on the front panel. I replaced the burnt tracks with some substantial copper wire. I think the relay contact overheated, melted the solder joints and the subsequent arcing burnt the pcb. I would expect better design from Electrolux but we all make the occasional mistake!
Russell.
Power circuitry - such as microwave ovens - tends to be designed using relays which do mount on the PCB, but their actual power contact connections are by spade lug to the usual heavy wiring.