If you use sealant in an inner tube, it will work best if the inner tube is on the large side for the tyre - e.g. a 32-38 inner tube in a 28 mm tyre.
Since the tube isn't stretched the thorn will do a fairly good job of plugging the hole it's just made in the tube, and the sealant doesn't have too much to do.
I've had a thorn in my tyre for a month or so on a couple of occasions, with the tyre needing pumping every 2 or 3 days until I got round to changing it. No sealant involved, but it could only be an improvement.
The trouble with pre-filled Slime tubes is that they don't come in a good range of overlapping sizes.