One of the considerations to answering this question is the business type, I mention this only because of the tax treatment of earnings. With an LLC or S Corp the owner pays taxes on the earnings of the business regardless of whether they are distributed or not whereas a C Corp pays its own taxes on earnings.

Having said that I think the best way to approach this is to set a goal of a fixed amount (and review that annually), in order to build that fixed amount up it usually a good idea to set aside a solid number like 10% or 20% of monthly earnings. I think it's also important to set this amount and be disciplined in funding it, rather than take the approach that you will just put whatever is left over at the end of the month, taking that approach often leads to nothing happening because people often find other things to spend the money on.