Will I know when it's time to meditate longer?

When I first started meditating I was very eager to progress. I started with five minutes and added another five with every week. I probably made it to 30 minutes or something when I just quit altogether. It was all quantity-over-quality. Towards the end there I lost a lot of will and motivation to keep going, I just wanted to do other things. I got bored and I let my mind wander and distract me, not making much effort in bringing the attention back to the stomach. Meditation became a chore and the only reason I kept going was because I didn't want to lose the routine, and also I hoped it was perhaps just a phase or something. In any case, to make a short story long, I've started meditating again now, 10 minutes at a time and since I have a cold and am tired all day as a result I don't become so restless. But when can I start adding more time? Will I "feel" it, or is it a question of discipline? Is it better to meditate poorly than not at all? If I add time, and find that I get really bored, is it because I'm doing it wrong or is it simply the nature of the mind?