How do I know if it is the right time to ask for a raise?

This is the hardest question. If you have been asked to take on more responsibilities you may want to bring up the topic.

You’re bored with current tasks

When you are a great colaborator and the numbers confirms that.

When you are above company goals that are set for yourself.

Taking on extra responsibilities

How long has it been since your last raise, am I good at my job and are there more people waiting to take my seat?

When your gut feeling is right, you’ve done your homework on your marlet value, and you are truly prepared to quit that day if needed.

When you see your efforts are making an impact and you are being praised by your boss and others in the company. You can also ask for a raise when you've had a big change in job responsibilities where you have more to do, make more decisions, etc.

On how successful the company is and your role in achieving it.

Always the right time. Don’t give up

You determine your own value, if you feel undervalued don't suffer though that, ask for a raise or find someone (a business rather) who sees you're value as you do.

If I've accomplished something significant, be it a project or task that I can point to as reason for the raise request.

Sit down and honestly ask yourself have you done a good job in your current role? Have you went above and beyond to help others? What are other comparable roles paying? What are your colleagues making(if they are comfortable answering) If you get a positive response on all these questions, catch the boss on a good day and let him/her know.