Ian said:

The Rev Dodgson said:

Divine Angel said:

The intended answer (I presume) is pretty easy. But there is not enough information to answer the actual question.

It depends how long they spend on the pitch before the start and after the end, and how long the exchange process takes, and how “the pitch” is defined (does sitting on the side-lines count as being on the pitch?).

I’ll go for an average of about 30 minutes, assuming “the pitch” is the actual playing area.

I see you’ve gone pre-emptively pedantive. Good.

What then of red cards, injuries, riots, acts of god..?

Exactly!

Never too soon to bring in the pedantry with a question like that. :)