The letters are usually good for 3-6 months and the pre-qualification $ amount is determined based on your income (typically 2 years tax returns are requested) and current expenses. 

@Uju Anyanwu - Do you have any posted requirements / qualifications for application approval, such as a minimum credit score?

They will want to know how long you've been doing this and your qualifications.

Working on pre-qualification so I can really start shopping.I’ve been talking with a few realtors/brokers as well and have started getting some traffic with off-market deals.My next challenge is finding a few solid contractors.  

It might be worth checking in with small, local banks and credit unions which may offer greater flexibility in terms of refi qualifications than national lenders. 

Those are more properly called a pre-qualification and aren't worth the paper they're printed on. 

In some cases, they will give you a pre-qualification letter specific to a property and an offer amount; sometimes they'll give you something more generic that doesn't have an address, and has a max price that you're qualified for. 

Hey guys, this week I plan on meeting with a lender for pre-qualifications.

I ask because the last time I attempted to switch agents and spoke with my old lender for pre-qualification (the same one who lent for my primary residence for yet another agent) he asked why I switched agents, I told him it was because this agent was a friend/experienced investor, then he never followed up with me. 

If you had to use an alternative income documentation loan, or another type of non-QM loan that allows some other qualification such as shorter time from foreclosure or bankruptcy, then having a DTI over 43% or over 45% can affect interest rate.