-hh wrote:

a 5 hour flight?
10 hours?
15 hours?

There is another important variable:

Is this an overnight or daytime or both night/day flight ?

Daytime only, first class doesn't bring you as much value as a night
time or mixed flight since first class seats allow you to sleep in a "bed".

And obviously, the new breed of first class "roomettes" such as on the
A380 is worth a hell of a lot more than some "first class" seat on a
north american airline that is just a glorified business class seat.