How often should big secondary schools have fire drills?

In the UK it depends entirely upon whatever the local authority requires. The near-universal recommendation is for a minimum of once per term, preferably near the start of each term.

The drills are supposed to be monitored and timed and if any significant problems are identified with escape timings, evacuation routes, assembly points or any other important aspect of the drill the plans should be modified and further drills carried out as required. They should also be carried out after any significant building works.

In practice, the majority of schools have very well-established fire evacuation plans and problems are rare, so more than the bare minimum of one drill per term is the exception as they are very disruptive to a school day.

thanks, I live in Uk and my school didn't do one at term 1 this year. So i'm guessing they will probably do one soon.

In the US the standard is once a month. That is K - 12. If there were ever a real fire you and a lot of parents would be thankful for all those endless drills.

Once a month usually.