The longest we've flown with our baby was 5 hours. It wasn't always fun holding him the whole time, but he wouldn't exactly let us put him down, either. We've taken lots of other short-haul flights and never got him his own seat.
We're flying to Israel this summer. It's a longer flight and we're deciding if we should splurge on the ticket.
What factors influence your decision on whether to buy a seat for your kid or just hold them?