I always leave the plastic on patches, it helps cover the areas of tube which have dried glue on them that are larger than the patch reducing the risk of tube stick to tire.
The 8 oz containers as noted last at least 5 yrs with periodic use before the remaining glue gets too thick to use. It could be thinned
with hexane/heptane/octane (ie gasoline) but it is easier just to chunk and buy another.