Normally five years . 2.5 yrs seems too soon but it shouldn't makwe too much difference -unless you think you have a really cheap deal in which case hold out for five years. If on the other hand you have got a so-so deal and rents keep falling you'll be annoyed at the five year term .
Mind you,if at any time you believe that the rent is excessively high compared to other shops there is now -or soon will be- a procedure in place that allows for arbitration at any time. Its complicated but is now much fairer for commercial tenants.
This is a new law and maybe other posters can clarify better what I mean.