There's no published percentage, and it genuinely varies by country and airport, but the honest answer is that most travelers clear customs without a physical bag check. Many airports x-ray all or most arriving luggage in the background, though, so your bag may be screened even when nobody opens it.
What raises your odds of being pulled aside: arriving from a flight route associated with smuggling or agricultural risk, traveling with lots of luggage relative to your trip length, visible nervousness, inconsistent answers to basic questions, prior customs violations on record, or detector dogs showing interest in your bag. Bags packed with dense, hard-to-read contents on x-ray, like lots of foil-wrapped food, also invite a look.
You can't control random selection, but you can control the rest: declare honestly, keep receipts for expensive items, pack food where it's easy to show, and answer questions plainly. Travelers who declare are usually processed quickly and courteously.