An alarm clock. Set for the time that your meditation is due to finish.[
No idea what kind of meditation you do, like, but when I was a professional in the spirituality industry, one thing I used to do was pay acute attention to the ambient sounds going on around me (cars in the street, water in the pipes, blood in the ears, etc).
As with all meditation, the main thing is that you stick to the time you've put aside for yourself, not what you actually do while you're doing it.