Take your first 3 months turnover as a guide if you are billing less than £10K. If you are turning over on average of 10K per month you should apply straight away to be registered from month 6.
Seek accountancy advice, the only ones truly qualified to advise on your business as they will have the true facts..think of their fees as an investment (in fact my accountant actually saves me more then they cost so their services are technically free)