Tatiana, you came and asked a very good question, and one which your open water class did not prepare you to answer, as most open water classes do not.
There are tools to allow you to answer this question, and quite a few people above have pointed you to those tools. In addition, your question has been answered, with the calculations to show the basis for that answer.
The responses are getting testier, and there is a reason for that. The attitude of, "I don't have to plan my dive, I'll just follow the DM," is one that gets people into trouble all the time. I highly suggest spending an hour or so reading stories from the "Near Misses" subforum. You will see stories of people signaling dive guides that they are low on gas and being ignored. You will see stories of people being led below the limits they agreed upon before the dive, and people being led into overhead environments for which they were unprepared. You simply CANNOT assume that the person leading your dive will do the work you ought to be doing to ensure your safety.
Dive planning, especially if what you are doing is diving off warm water charter boats, isn't very complicated. The dive briefing should give you a basic description of the site, and idea of the planned depths, and a mention of any hazards. You have only one kind of tank to sort out, and that's the standard Al80. It's easy to remember that that tank has 2.5 cubic feet in every 100 psi of gas. Pick a rule of thumb for gas reserves --
HERE is a cheat sheet that uses a standard PADI ascent as a model -- and agree with your buddy that you will start your ascent at that point. Stick to your guns! A quick discussion of hand signals, buddy separation protocols, and how you share gas will be a pretty good dive plan for the kind of diving you appear to intend to do.
You are getting harsh answers because your responses worry us. Ceding responsibility for your dive gets people into trouble, sometimes serious trouble. We are trying to encourage you to step up and become a strong and independent diver. You will have more fun, be safer, and stay in the sport longer that way.