Aim for a 90 minute nap, or some other multiple of 90 minutes.
The reason for this has to do with sleep cycles. In the average person, the first 65 minutes of sleep don't really do much for you. Yes, they make you feel less tired, but that will wear off quickly and your brain is still going to operate at the same tired level that you had before your nap. You are spending those 65 minutes in the first 4 phases of sleep. They are more akin to your brain powering down, rather than your brain repairing itself.
After you have done the first four phases of sleep, you move into the fifth, REM sleep. This phase usually lasts about 20 minutes and this is the deep, restful sleep that actually repairs the brain from all the work it's done over the day. The final phases of a sleep cycle is 5 more minutes of non-REM sleep that helps to gradually bring your brain back to working mode. In many cases, people will actually wake up very briefly during this time (if they are sleeping during the night), but they won't remember it because it was such a short wakeup.
Each sleep cycle you go through will actually increase the length of your REM sleep phase. Your first cycle may only have 15 minutes of REM, while your fifth cycle might have half and hour of REM.
These sleep cycles are key. If you wake up at 30 minutes, you have basically just wasted half an hour laying unconscious that did very little for you. If you wake up at 60 minutes, you will not only have wasted an hour, but you will feel groggy and lethargic because you were in a much deeper phase of sleep. Waking up during the REM cycle is the worst; you're causing your brain to jump from fully shutdown to fully on in a matter of seconds. Sleeping for 2 hours is bad for the same reason that sleeping for 30 minutes is bad; you've completed one cycle, and now you are interrupting a second.
Completing a sleep cycle is so important that if you allow yourself to fall asleep with no alarm to wake you up, you will most likely wake up at a multiple of 1 1/2 hours. You might wake up at 1 1/2, 3, 4 1/2, 6, 7 1/2, or 9 hours, depending on how tired you were when you went to sleep. This is why many people feel that they actually are the most awake awake 6 hours of sleep, but think that 8 hours of sleep makes them more tired. It's not because their body actually functions better with less sleep, it's because at 6 hours, they are waking up at the end of a sleep cycle, while at 8 hours, they are interrupting a cycle which makes them feel groggy.
On a side note, don't forget to take into account the time it takes you to fall asleep. If you usually take 10 minutes to fall asleep, set an alarm for 100 minutes, 15 minutes to sleep, 115 minutes, etc.