I have just applied cement on some stone joints, and I was wondering for how long should I pour water on it? How frequently should I apply water? Is it twice a day, or thrice a day? I really need to avoid cracks and weak joints.
Best is to keep it damp "forever." Design strength for most concrete is based on 28 days at 21C/70F. The early part of the cure is the most critical. Time is doubled at 50F/10C and reduced at higher temperatures - so long as it's kept damp.
Covering with damp cloths/burlap/sacking and then covering that with plastic can help to keep it damp longer (from a single application of water.) Just covering with plastic helps some. How frequently you need to apply water will vary with climate and how much you have done to keep the water in place.
Procedure which Ecnerwal has explained is "by the book" and I agree 100% if we would be speaking about concrete slab, wall etc. But since you are just using cement for joints in a stone wall no need to pure water over it.In fact, you could cause damage if you would pour water because there is a chance that cement would wash of. My advice would be just to use "heavy duty" cement plaster for this