The exact length varies by lender rather than by any single rule, so the number on your pre-approval letter is the one that governs, not a general industry figure. Some lenders offer a shorter rate hold with the option to renew, others a single longer window.
If your pre-approval lapses mid-search, the fix is simply going back to the lender to refresh it, which usually takes a day or two and may reset the rate to the current one if rates have moved.
Timing your pre-approval to roughly match how long you expect to shop avoids both a lapsed letter and a rate that locked in too early.
Get your hand-reviewed valuation from Joel Dyck.