In the UK, is there a certain amount of time after which the company is happy with the candidate and hands over the commission to the recruiter? Or does that vary from company to company? What would be a typical amount of time?
This is entirely down to the contract between the recruitment agency and the company. I have seen some that are a finders fee and paid immediately, others that pay out once the new recruit has passed their probationary period, and yet others that pay an element up front and the rest later based on success criteria.
The most common that I have seen from agencies is payment on acceptance of contract. This is logical, and is probably the best compromise between both sides. The recruitment agency would like to be paid when they send a candidate for interview, and the company would like to leave it until the candidate has proved themself and passed the probationary period, but those two extremes benefit only one party.