It can take as fast as a week, or on the longer end 35-60 days once you find the right house.
For most people I would suggest getting prequalified about six months in advance if possible. That gives you time to clean up any negatives on your credit report if any, pay down balances if you need to, save for a down payment if you need to, save for closing costs if you need to, and then start checking out general areas, drive times, neighborhoods, etc.
Then about 3months before you want to move, start looking inside homes, narrowing down what you want, don't want, can live with, can't live without, etc. You'll also start to get to know prices and quality and pick the deals from the not so good deals. This gives you a comfortable 30 days to pick the right house and up to 45-60 if you really need to look more. About 30days out from the day you want to move, you need to have locked down a place and be moving forward with contracts.
You can search for Irving homes at the link below.
Let us know if we can assist you in any way with your Irving home search.