Normally when the SG is stable over a couple of days - normally 1.012 or lower, depending on brew. Most of my coopers kits finish around the 1.010 mark, but my AG brews go down to 1.006 - it all depends on the yeast supplied with the kit, and what fermentables are in it.
As long as you are sure it's finished fermentation (i.e. it hasn't been in the cold where it could have got stuck) and you use decent quality bottles - no cracks or chips - you should be fine.