The gas furnace takes a lon time between heating my home. It will come on for about 15 to 20 minutes. Once it cuts it will take uo to 30 minutes to come back on. What is causing this to occur?

That might be normal... how much does the temprature drop before it comes back on... thats a better indication of a problem.

Also what kind of furnace is it... Heat pump, electric, steam, fuel oil, natural gas?