If I get pre-approved for a mortgage, that's generally good for 60-90 days, but what if I don't find the right home in that time frame? Another hard-pull on the credit to get pre-approved for continued pre-approved-status searching? Or will sellers potentially take the expired pre-approval (and assurance, which only goes so far, that nothing's changed for you financially) as evidence of your ability to finance the stated amount? I assume the latter is better than not being pre-approved at all, but obviously not better than having a current pre-approval... But I also don't like the aspect of getting rushed near the end of that timeframe if I don't find something I love just to keep from getting approved again and again...
As mentioned before - you're over-thinking the hard-pull issue. But do try to make the preapproval as close to the actual bidding as possible - because it costs money. At least from my experience, you'll get charged the application fee for preapproval, while "pre-qualification" is usually free.
If you're seriously shopping, I find it hard to believe that you can't find a house within 3 months. If you're already in the process and your offer has been accepted and you opened the escrow - I believe the preapproval will be extended if it expires before closing. I've just had a similar case from the other side, as a buyer, and the seller had a short-sale approval that expired before closing. It was extended to make the deal happen, and that's when the bank is actually loosing money. So don't worry about that.
If you haven't even started the process and the preapproval expired, you might have to start it all over again from scratch, including all the fees. The credit score is a minor issue (unless you do it every 2-3 months).