Hi
A good question.
Starting the markets refreshing at a few minutes before the start of the game is good, but the problem with football markets is they are monitored for a long period, and you will find peak times where the number of markets is massive, e.g. Saturday afternoon.
It also depends on your refresh rate. Here are some numbers you can consider:
Betfair allow you 20 free data requests per second.
20 markets refreshing at 1 sec = 20RPS
40 markets refreshing at 2 sec = 20RPS
etc..
bets update (weighted as 5 requests ) every 3 seconds = 1.7RPS
statement update (weighted as 5 requests)
market_score update
I suggest you start with a smallish number of concurrently refreshing markets - say 10, and gradually add some more, until you find the point where the program's response starts to drop off, or the market refresh rate starts to increase.