A few things to keep in mind in having this discussion:
How much of a raise are you wanting: 1%, 5%, 10%, or 50%? There may well be that question to be asked of you, "How much of a raise are you wanting?" that may be where you'll have to give a number and thus it is a good idea to be prepared for this point.
Justification for the raise. Surviving 14 months on the job isn't that good of a reason for claiming that a raise is justified. That your skills may be better, you know the company more, or other details of increased value you deliver now compared to when you were hired may be much more useful points here.
Don't forget that the financial health of the company may be a part of this conversation. If the company is a start-up then raises may be hard to do in terms of straight money though stock options could be used as a way to reward a faithful employee in a sense. On the other hand, if the business is going great, then it may make sense to want some kind of profit sharing possibility and just see what can be done.
If the work environment is rather laid-back then I may approach my boss and ask about how would I go about getting a raise and go from informal to formal conversation on this point. On the other hand, if the company is more formal then I'd schedule a meeting and ask the question of when does my salary get reviewed against my performance as I'd like to discuss updating my compensation package please.
No, it isn't too early though understand at this point you want to know what is the process for salary negotiations as you've been there for over a year and believe you are worth more now than when you started. Most places I've worked had a 3 month probation on the job and then annual reviews after that. As you are approaching what would be a combination of a year and a probation period, it isn't a bad idea to inquire as to what is the process for updating the compensation.
As for the 15%, the key point is how well can you justify that kind of increase. If you believe that your skills have increased greatly, that you know the business much better, or other major productivity boosts have happened then you could probably justify this boost assuming the company has the money to handle the raise. A key point can be to consider how well do they think of your work and is it worth upping your salary to get more out of you? I can remember in my first year of working that I had a few raises though this was because of changes within the company as well as being in those dot-com boom days where I worked as a Software Design Engineer in Seattle.
