How long do you think that will last?
I use 20-30ml in a small medicine cup, once a week, so 250ml should easily last at least 10 weeks.
Also do I just leave the sensor in the tank or do I need to extract a sample from the water and place it into the sensor.
Have you used glass pH electrodes before (the handheld kind, possibly)? If not, I would strongly encourage you to do a google search and research them. Anything left in the tank is going to quickly acquire a nice layer of scum within a few days, therefore requiring frequent cleaning. How much the algae growth affects your reading, I couldn't tell you, maybe not at all. (In my situation, accuracy mattered greatly.) And frequent cleanings run the risk of damaging the sensitive bulb. If it were me, I would get two probes and keep one in the tank and properly store the other. When it is time to take your measurements, place the clean probe in the tank and let it stabilize for a minute, then compare the two readings.