I'll try to make this answer simple.

Your pre-approval lasts usually 90 days.

The reason: your credit report.

Your credit report is usually good for about 120 days before they need a new report.

If you get under contract on a home on day 90, you'll have about 30 days to complete the mortgage, or you'll need a new credit report.

Each day you go beyond 90 days, it makes it more and more likely that you'll need a new credit report.

So to be safe, if you're still actively shopping at day 90, just get a new report and refresh that approval.

Can my pre-approved amount change?

If you've kept your profile the same: same job, same income, same amount saved up for a down payment, your pre-approved amount could still change.

Rising interest rates.

Rates can be volatile.

Blame investors. Investors buy mortgage-backed securities, and the demand for them go up and down depending on where investors want to put their money.

The 10 year treasury bond is a similar investment vehicle to a mortgage-backed security. So if you see rates on the 10 year go down, you'll see rates on mortgages go down.

Investors can react to inflation news with fear.

If you're trying to buy a home and suddenly an inflation report says "inflation is red hot" then you could see mortgage rates shoot up.

That spike in interest rates will cut into your purchasing power.

If it's drastic enough, the lender might call you back and say "remember how I said you're approved for $500,000? Well that's gone down to $480,000"

new debts

If you buy a car while your shop for a home, your pre-approved amount will drop.

Other reasons

if you have a change in assets, income, or employment, you're pre-approved amount could change.

when your pre-approved amount goes up

If rates go down, you will see your pre-approval amount go up.

If you save an additional $10k while you are home shopping, then your approval will go up about the same $10,000

If you get a raise while you shop, you could see your approval increase.

Here's an example: you're trying to keep your mortgage payment at 30% gross DTI. If you get a raise that gives you $180 per month more in guaranteed salary, you could technically afford about $60 more in payment staying at the same DTI ratio (debt to income ratio)

$60 more on a payment is about a $10k increase to your loan amount.

I hope this helps!

Basically just keep an eye on interest rates/10 year treasury as you shop. Notify your lender about any changes to your savings/employment/income.