Around here its OK to do it for a hundred yards or so, but then come
past in a way that offers to return the favour (i.e. move by steadily,
make eye contact, say hello). If the other person doesn't want to latch
on, that's their business. That said, when knackered and facing a
howling wind, my definition of a hundred yards may get a bit extended.
And there are times when I go to do my turn and realise I can't even
pass the person in front!
IMO, catching somone up then sucking their wheel is much more of a
no-no than jumping onto the wheel of someone who happens to pass you.
If you catch someone up intending to offer to work with them, it's OK
to catch your breath before offering to do a turn.
On an organised ride (of which we have many around here), the
presumption is that drafting is OK - it's like you are all members of
the same club for a day. True happiness is latching onto a couple of
tandems for a few miles and flying past everyone else! Tandem riders
around here seem to enjoy towing trains of people on singles - I guess
it's an ego boost.