it sets pretty fast, but i would let it cure at least 24 hours before adding water, preferably 48 hours. as for holding in a plug, it should, but a better option would be to replace the bulkhead with a threaded bulkhead, and then get a screw in cap so you can be 100% sure it will not pop out accidentally. if your sump is already drained, this is the perfect time to switch bulkheads.