I'm planning a trip to Ireland later this year (me + one other person) and am considering using points to travel in J. The first two options that I've found have connections in Europe (so J is in a longer, 10h or so segment), while another one has both segments split pretty evenly (~4h30m and ~6h30m). The latter also happens to have the lowest fees out of all three options I've found so far. While it's nice not to pay more fees than I have to, it does make me wonder whether we'd get much value out of the J seats considering the shorter time spent in them.

Generally speaking, how long does the flight have to be before you'd consider upgrading? In other words, is there a minimum length where you feel that you're getting good use of the extra amenities?