I usually use a timer, but there are periods of time when I don't want the jarringness of a sound to end it. Then I'll use a mala, moving one bead for each breath. You can use it with your timer a few times, and get an approximate idea of how many cycles your standard meditation length is. If you use two of the short bracelet style or a Jodo Shu style juzu, then each time around the first string is one bead on the second string. You can set your time by where you hold the second string at the start of your session - hold it so that you will count the correct number of beads before reaching the guru bead.

For really long sessions or when counting short mantras, use the hanging beads to count times round the second string.