Relays are stressed when they are switched. The worst case is switching OFF a DC load with high inductance. The inductor wants to keep the current flowing and it will cause a spark to jump across the relay contacts while they are opening.
That's why relays rated for 250V AC can only switch 32V DC. The AC voltage will go to zero in a 100th or 120th of a second, extinguishing the spark.
Leaving the relay on all day is best for it. An SSR in the same duty cycle will stay hot all day and that's why it needs a heatsink.
The relays on the cheap boards are probably OK for 6-7A. However the PCBs may not be designed with that in mind and they are almost invariably not designed for mains safety. Look for something that has a very clear high-voltage area delineated on the PCB and nothing from the LV side of the board crosses into that area. Also look for strong connections that will withstand mistakes like you dropping the object and grabbing the power cord to stop it hitting the ground. (You should also have good strain relief at the cord-entry point on the box.)
[Edit: Fixed my math on the AC line frequency.]