First of all, congratulations on the revenue and from your statement that it is not even close to failing I infer that you must have at least 15% to 20% profit

First of all, congratulations on the revenue and from your statement that it is not even close to failing I infer that you must have at least 15% to 20% profit margins and have replaced salary and benefits you personally would be earning working elsewhere. Working from that scenario it seems that you have taken out a significant portion of the risk. That is you have built/created "X" it is not just a great dream. You have proven that "X" is a business by convincing people/businesses to pay for "X" at a price that produces a profit. Your risk in bringing on a co-founder is that in trying to grow the company he will screw up the current secret sauce or distract you from what has worked to get to this point. My own experience has led me to believe that many people who say they can deliver high growth to early stage enterprises cannot. In some cases it is because most of the growth they take credit for in another firm had little to do with their personal performance and much to do with firm brand/reputation, well established sales processes and other intangible factors. They get into this new environment and cannot reproduce former results. Other times, people in sales and marketing are much better at selling themselves than they are at selling the product. And, then there are those that really deliver results! How can you be certain this potential co-founder will really deliver growth? Other comments point you in the right direction. If you decide to go ahead, then vest over 4 years - that is very common. Also I would make some milestones around sales growth. If certain growth targets are hit, this person can choose to take a cash bonus or to accelerate vesting. Finally 25% seems high to me since you have taken out so many of the early stage risks - something more on the order of 15% seems reasonable. Of course, be wary of accepting advice from strangers on the Internet.