How to determine the end of a meditation session without a clock? I have a clock, it just is distracting having to look at it every few minutes.
If you have a smartphone, consider using a meditation timer app for keeping track of sessions. Simple ones like Meditation Helper (Android) or Meditation Timer (iPhone) are free and do the simple job of keeping the time, ringing a bell when finished. This cen perhaps let you just focus on the meditation for the entire session.

Note: I only have experience with the Android Meditation Helper, which is why the link is provided. It has reminders and a log, quite simple.

UPDATE regarding the use of timers and bells:

Using a meditation timer can be beneficial, especially for beginners, where hearing repeated bells at intervals (starting out as frequent as 2 minutes, increasing to 5, then 10, and so on) can work as a reminder to gently return to the meditation object. However, there is a point where the bells become a hindrance, where they may disturb a deeper meditation, so the recommendation is to phase out the use of the bells. 

Meditate for as long as you can, each time. Only if you have limited time for a setting, then use the bell as a reminder. The app works well for this, where you can first set the bells at intervals, then to end the session. If you only look at the meditation timer (locking the screen during meditation) after the meditation session has ended, you can use it to evaluate the session. You may find that sometimes a 30 minute meditation can be challenging, and other times you're surprised to find that an hour and a half passed while in deep meditation.