I would encourage you to meet with a lender for preapproval right away. If there are any issues with your credit, work history or assets, the sooner you find out the better chance of getting things right in time to meet your deadline.
The other reason is because things change. Prices in many areas are going up, rates have been trending higher and all of this means less buying power for you. Should you be in a good position with your preapproval in hand and the right situation presents itself, you are positioned to take advantage rather than scrambling and missing out.
Let your lender and agent know your intended time frame up front so they are prepared. If you were in my market and going to be my client, I would send you listings to help you see the market and learn how fast things are selling, the pricing, the trends and help you get a sense of what you can afford early on. There is nothing worse than getting your hopes up only to be disappointed that you can't quite qualify for a home you love.
Get started sooner rather than later, it will make for a better experience.