How long will I be able to keep picking my rhubarb?
My Vicky finishes around september ;D
As long as you like but never denude the plant completely.
Eventually, depending upon how fast you use it it will become a bit 'stringy' then you might not want to use it.
Similarly if it runs to seed it will alter in taste & texture!
It gradually becomes less juicy as the summer goes on. Stop when it no longer suits your taste.