I was charged with an offence in July 2010 and got a summons to a first court date in Sept 2010 then plead not guilty and was put off till Feb 2011. How long can it take before it is too long.

Depends on the Justice of the Peace and the jurisdiction. In my experience, ten to twelve months seems to be the common time frame before a stay of proceedings will be granted.

Keep in mind the Court will look at factors for the delay. If it's because of a backlog in the Courts or an inability to proceed on the part of the Crown, the stay is more likely to be granted. If you've requested any adjournments for more time to prepare or waived your right to an earlier trial, the request will probably be denied (so don't postpone your trial hoping for an 11b).

To the best of my knowledge you have to file a motion with the Court prior to your case being heard for a stay to be considered. You can't just bring it up spur of the moment if your trial isn't going well.

In addition to the delay, at least 10+ months of which has to be attributable to the crown or institutional delay, you have to be able to show how the delay prejudiced you as well.