You ask how durable is an 18k gold plate. Wow, I do wish I could give you an answer in months or years or in any time frame. The fact is, without knowing how thick the plated surface actually might be there is little way to predict wearability. Even if I knew the thickness, the best I could say is, "It will wear better (or worse) than this or that other sort of plate or layer."
So much depends on how often the ring is worn and the conditions of that time when worn. I do believe you will take good care of the ring and that does make me happy. Too many folks will come into our business with a worn out ring, packed with dirt and stones dull from all of it … then proudly say, "I never take it off." What a mistake they make! Gentle and regular care and cleaning is so vital to keeping a ring looking the best and detecting if any stones might have worked loose.
When the ring is worn, if your hands are in contact with surfaces which contact the ring, daily wear will eventually take the plate from that contact area. This wear might come from a desk, keyboard support or simply items you normally handle in day to day work and living. Being more alert to what the ring touches will certainly give you a "heads up" on preventing wear from everyday contact.
The National Gold and Silver Marking Act (USA) requires certain thicknesses of gold plate for a jewelry item to be stamped as gold plated. The mark might be "gold plate", "hge" for heavy gold electroplate or simply "plate" or "plated". These marks indicate a recognized thickness of gold plating. If there is no mark saying gold plated in one form or the other, probably the plate is of the thinner sort and not thick enough to qualify for marking as plated. Sure, it is plated but not thickly enough to warrant the marking. If not marked as plated, the ring is probably plated about as thickly as typical costume jewelry and will wear about as well.
How long? I simply can not say. My wife wears lots of Avon jewelry items all with only a wash of gold color on them. (She happens to like it ..) Still, it is surprising how long the surface does last. I suspect your ring has a thicker plate than that and should last longer.
When caring for the ring, use essentially the same steps to clean as with CZ's. However, do not often use a polishing cloth, especially the chemical or rouge treated ones. These are made to remove tarnish and are capable of ever wearing away the gold plating on the ring! Wipe gently with a soft cloth only to dry from cleaning in a mild detergent solution. You may use a soft brush to clean behind the stones.
The surface will show some wear from daily contact and that is normal. This will happen long before the plate is worn through. Trying to polish out the wear from daily contact will only help remove the plate and greatly reduce the life of the ring.
I realize this answer is vague. There are simply no fast rules as to how long a plated jewlery item will last since each person has a personal life and conditions are different.