All rent to own contracts are different. Some will give you 12 mos some 36 mos to get your finances in order and apply for your own loan. Not all contracts will give you a rent credit either. It is at the sole discretion of the person who owns the house. If they do give you a rent credit and you are able to buy the place in the time allotment they give you, the rent credit will go towards the purchase price as part of the down payment. Also, any down payment you had to pay to get in the house (some are as high as 3% of the purchase price) may also go towards the down payment.
Rent to own scenarios fall apart a lot of times, and never come to fruition. The buyer never seems to get the finances or down payment together, or they still cannot get a loan due to their credit. Only about 20% actually buy the home they were renting to own.
The main thing you would want to do is set the price NOW of how much you are willing to pay for the property. Don't wait for 12 mos to 3 years. (not that values are going to skyrocket, but it's better to set the price now) Get everything in writing and have it looked over by a real estate attorney. Also make sure that the owner is still held accountable for any major repairs. Why would you want to replace a water heater or furnace or other huge ticket item in a house you may or may not ever own?
Remember also, that if you cannot secure financing, or decide you don't like the house afterall..that you lose your deposit and all the rent credits. That is the penalty usually for not exercising your option to buy the home.
Rent is usually a bit higher than the market rate, only because of the rent credit. I would say on a house that would rent normally for $1000 mo, you might be looking at as much as $1300 to 1500 a month. It all depends on the owner.
Good luck and hope you find a house you like and are in a financial position to buy when the term is up!