I always use a timer to start off but sometimes will switch off the timer and continue meditating for an unspecified amount of time on days where I feel up to it.

Without a timer I tend to start wandering off into thoughts about whether or not I've been meditating long enough, and I find that incredibly distracting from the process. When I know I have a timer/alarm going off at the right length of time, I can put my mind to rest and not think about how long I've been meditating.

Also if I'm getting back into meditation after a period of not keeping up with it, I sometimes break my meditation down into smaller chunks. Even though I meditate for the same length of time, having alarms spaced closer together makes me feel like it's taking less time, so eases me back into longer sessions.