How long should you give a horse to become a winning one?

How long is a piece of string? . Technically if it's a handicap horse you could always win one whenever if it got very well handicapped. For listed/group races if you haven't won in the first 4 starts and you're sure of the horse is running at the right distance then probably unlikely though conditions could play a huge part in this one.

Thanks for the reply, whats the best way to get a good horse though?

I've paid around £400,000 for 2 year olds who turn out to be shocking

Breeding is really the only sure fire way. Buying unraced 2YO's at auction is always a lottery.