Without special clothing (desert clothing) you need to take a drink every 90 seconds. With a full set of desert clothing, one drink every 120 seconds; so a full waterskin will last 6-8 minutes.
A better alternative is an enchanted water tiara (it has quest requirements), which is practically unlimited: each "drink" reduces it by one charge, one charge requires 3 water runes, and you can add as many charges as you want.
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a few minutes.
they can last anything from 1-3 Minutes before one by one they go out.
I can't find specific information, but it will be much less than combat with most monsters. Normally you should have time enough to get to safety. On the other hand, your health will gradually go down, so better take enough waterskins with you. Those will last much longer than food.
It depends how long you are on Runescape.