How active are Force ghosts?

I think it varies with the "ghost", but they do eventually pass onto the next life for good. They might be able to be "summoned" or just appear when they want to. It probably takes a lot out of them to appear to the living, so they wouldn't constantly be pestering Luke.

The force ghosts are as active as the plot needs them to be. lol

Not trying to be glib about it, but there is not enough activity from the ghosts to get any more answer than that. we also don't know what kind of power they actually have or how much of their will remains in ghost form.

Obi-Wan ghost appears it seems when Luke really needs him and gives him just a little bit of direction. But then he appears to be able to talk to Yoda at will. So we really just don't know.