At the moment, I think we can assume that the OP and the supplier are in a
position of informal discussion regarding the item (they've said "post it
back to us", and he's said "sure, just send me a pre-paid label and I'll
drop it into a postbox"). So the clock hasn't started ticking yet.
If they don't respond to informal discussion with a suitable solution, or if
they continue repeating their request for him to return the item at his
expense, that's the point at which it would be appropriate for him to send a
formal notice that he requires the sender to collect the goods within a
certain period of time (21 or 28 days seem to be the rule of thumb here). It
would be sensible to include with this a copy of any previous communication
in which he states that he will accept a pre-paid postage label as
fulfulment of this obligation (assuming the item is small enough for him to
send by post at no significant inconvenience to himself)[1]. The notice
should explicitly state that if the item is not collected (or postage paid)
by the end of that period then it will be presumed to have been abandoned
into the possession of the recipient.
That's the only way that the recipient can create a definite cut-off point
at which he is entitled to assume that the sender no longer requires the
goods back. If he simply lets them go quiet without serving a formal notice
of collection then it could be months, or even years, before it gets to a
point where a court would agree that the goods had been abandoned.
[1] For avoidance of doubt, the recipient is under no obligation whatsoever
to arrange return postage, even with a pre-paid label. He can simply require
the sender to collect the goods (or send a courier to collect them). But if
the item is small enough to post, it's probably less inconvenient to do so
than waste his time sitting at home waiting for someone to arrive to pick up
the item[2].
[2] If the sender arrives to collect the item within the deadline, but is
unable to do so because the OP is out, then that would, at the very least,
reset the clock, and could potentially make the OP liable to pay for the
goods and/or reimburse the sender for their costs in making the unsuccessful
collection attempt.
Mark