I always pay within a week, I feel that's an acceptable time.
My two friends though, are very lax about this. Last year I booked tickets for the 3 of us in July, for a show in November. Although we all saw each other several times in this period, they waited until November and paid me at the theatre!
We're all seeing a show in 2 weeks time, that I booked at Christmas. One of them (who lives in my village!) has mentioned a few times she'd put the money through my door but hasn't. The other one lives a bit further away, but nothing to stop her doing a bank transfer!
It's not just me who books things, they sometimes book group tickets too but their timelines for payment are very different to mine!
The tickets aren't mega expensive, they tend to be about £30 each, but I'd rather be reimbursed soon afterwards. Months later is taking advantage, I feel.