There are good answers in this thread already, To add a new concept, the amount of water depends also on the glider. The newer gliders, particularly 18 m hold water well in weaker conditions. The older gliders, not as well..

The 2-3 kt rule above is a really good starting point, but it really depends on the nature of the day.

The most important consideration is "can you climb" if you can't you are too heavy and that makes you circling speed higher and your radius wider so you climb more slowly because of the combination of the two factors (speed = drag, radius = weaker lift).

This is where the nature of the day comes into play. Are the thermals narrow? Are the thermals bubbly or turbulent? Are they smooth and wide? These conditions also affect the amount of water that should be carried.

I remember many years ago when racing in an ASW-20, I was heavy with water and bombing along between thermals thinking I was smoking. My buddy in his 1-35 (dry) was keeping up. While I was loosing him on the run he gained it all back in the climb. I learned a good lesson that day. Heavy and fast is not always right. The time spent climbing is just as important as the time spent running. Both have to be minimized.