# What should $T$ be when gamma trading?

I am studying some gamma trading, and there's one thing I don't understand. On the option we are hedging, what should its expiry $T$ be, and for how long should we do the trade?

What I mean is, is it normal to do the trade until expiry $T$, or do we usually only do it in the period $[0,t], t <<T$ and then at time $t$ we stop?

What should that decision be based on?