I re-use my potting mix in planters year after year after year, but it is
NOT "compost", and I wonder if we are writing about different growing
media. Are you in the US or elswhere?

Thanks to everyone for the replies.
I'm in UK where "general purpose compost" is described on the bag as
"peat reduced". Reading on (and maybe I should have done that before
posting the question) it says it contains enough nutrients for the
first 4-6 weeks only, after which you should go and spend more money on
liquid fertilisers, so it looks like this stuff is "soilless".
I'll try recycling with the time release granules. We do get a lot of
ants in our window boxes so it may have to be dumped anyway.