There really is no "limit" to how many calories that a person can absorb in a day. Using them, however, is where other factors come into play, that are very much dependent on the person. For example, during his heaviest training days, Michael Phelps is reported to be consuming between 10-12,000 calories a day. However, if you don't exercise to meet that level of intake, it's going to get stored as fat.
There are going to be a couple of factors that determine how long/far you can go on a hike. The first is fitness. If you only ever train up to 3 or 4 hours for hiking, then you probably won't be able to hike much beyond that before your muscles are just too fatigued to continue. Someone that trains for a 1/2 marathon, for example, probably wouldn't be able to just up and run a 100 mile ultra. If you are unable to keep hiking/climbing at a certain point, compare that to your training and see if it is nutrition or fitness.
As far as the eating/absorption, most people can absorb between 250-400 calories per hour while exercising. Dumping 1800 calories into your system twice a day may not be optimal for you. This is something that you work out in your training, how much you can eat "on the go", and how your body reacts to the large influx of food/calories.
There are numerous calorie calculator and activity cost calculators around on the web, I would take a sample from several to get a "general" guideline for your activity cost, and if you have a 5 hour day planned, calculate your climbing costs for those 5 hours, aim for an hourly intake of a certain percentage of that, and get the rest from your meals.
