I don't plan on taking them off, except when I compete in a triathlon, where they are usually not allowed
not sure that that seems right...
i've used aero bars in 2 of the triathlons i've done (i'm an occaisonal triathlete) and that's why i bought them (increased speed in tris)
they are NOT allowed in most mass-start races like criteriums and many road races --- i used to take my off once a week for the local crit race - as they could interfere with other riders or cause a problem in a crash.
in a club ride you could probably have them on the bike (unless they have a rule against them) but probably not a good idea to use the aero bars in a paceline - maybe if you're at the front, but otherwise seems like a recipe for a crash.
otherwise, you can ride in the aero as much as you can... but it takes practice and i am not good enough to turn corners or even ride some tight curves... in the city it's a bad idea as breaking and quick manuevers are not possible. on mine i usually only get in the aero bars on a long stretch of open road, but many practices triathletes can ride most of the time in them.