Five years is fine, but they should only concentrate on TES, forget about Fallout =P
Pretty sure Fallout is more popular then TES is. That means they are more likely to forget TES and concentrate on Fallout, so be very very careful what you wish for.
bliviongate:
As for the OP: Five strait years of development time is a lot, especially if they end up using the same engine. However, BGS likely was not working (at least not the full team) on TES:V during the whole five years due to Fallout 3's development. In all likely hood the game has only been in full scale development for a little over two years, with a small team working on it for who knows how long before that. So three years for a new game? Thats not bad.
What Bethesda really should do is have two full time teams working on Fallout and The Elder Scrolls. Both games have become very popular, and could each justify its own dedicated team working on it.