How long is a piece of string question, but I would look at the British Franchise Association's website.
In brief - what is the franchisor offering - in terms of equipment, training and on-going support. Will you have geographic exclusivity?
What will be the on-going costs? You will normally have to pay an on-going percentage of turnover and often a marketing levy as well.
What will you be expected to do in terms of performance - is there a minimum turnover. And what sort of standards will you have to adhere to?
What are current franchisees earning in terms of profit and turnover? Talk to them - don't just take the franchisor's word for it.
There will be a legal franchise agreement which you'll need to go through - if you have no legal knowledge then it might be worth talking to a lawyer.
You also need to do your homework - is there a demand for your product/service in your area? Is there a lot of local competition.
And put together a business plan.
When I started I contacted my local business link who had business advisors and who could help me with legal advice.
What sort of franchise are you looking at?