I have always kept car rental receipts longer than ANY others, because I have had the most trouble with car rental companies. I keep them at least 18 months.

Airline receipts I general toss immediately after the trip if all went well. If not, 6 months or longer.

Hotels I keep at least 6 months. There have been a few incidents of unauthorized or incorrect charges, including fraudlent tip inflation on behalf of wait staff.

Other purchases I keep until the CC is reconciled and paid. Usually a month or so.

Keep in mind though that all my receipts are scanned and saved on a computer by the expense company and by my employer for any tax reasons. So when I throw them out, I still retain a record just in case.