To consider how much you should eat above your current maintenance intake, we need to consider how fast you should be gaining weight, which is in turn dictated by how much muscle you can reasonably stand to gain. If, of course, you are starting from a point of being severely underweight for any reason, then the guidelines would be different, but that’s a job for a dietician.
Your rate of weight gain should be such that it minimises fat gain. It’s a very rare circumstance where fat gain over and above the minimum amount necessary is welcome by anyone – some exceptions may exist in heavyweight strength sports, for example, but that’s about it. Rather, the majority of people looking to gain muscle will be looking to do so to either improve in a sport which requires overall fitness or adherence to a weight class, both of which would be hindered to a large extent by the unnecessary addition of fat tissue, which could have been avoided, or simply to look good with their clothes off.
Because of this, it’s pertinent to get a good understanding of how much muscle it is likely for an individual to gain and at what rate. Of course, there will always be genetic outliers who can gain more than this, and there will be the unfortunates who gain less, but a good estimate for muscle mass gain is (as calculated by Alan Aragon in his manual Girth Control):