My wife and I always gift our F seats to her parents when we fly. It's probably happened 4 or 5 times over the past few years. We'll board with them, say our goodbyes, and then make 1 or 2 visits throughout the flight. If it's a 3 hr or shorter, generally one to say hello. If it's cross country or ocean, generally 2. Usually 30 seconds to a minute, how are you guys doing, etc. I'd feel stupid if any longer. I can probably count twice where I've gotten a look from a passenger, but I've never had a FA say anything. One time a seatmate said to my father-in-law "He should have used miles to upgrade and wouldn't have to visit you", to which he responded that I actually used cash to put them in those seats. Seatmate couldn't understand why I booked him in coach and me in F if I was planning on seating everyone in the opposite, laugh out loud.