I've been playing for over 30 years and if I haven't been playing barre chords for a while (a month), my endurance with them goes down.

To build a lot of strength with barre chords, play them a lot; find a song like haghj500 suggests and play it regularly. Once your endurance is up, make sure you play something with barre chords once a day I'd say to keep that strength there.

When I did the music store band thing the last time, every song we did invoved barre chords! My endurance was usually 30 - 45 minutes. Half way through I found I could go the full 1.5 hours and feel like I could still keep going.