I use a digital stop-clock that beeps every 30 seconds of whatever time I set it to count down from, so it not only makes it easy to keep track of, but adds to the Marine players feeling of being under pressure as they hear the alarm going off (a push of button mutes the alarm and continues the countdown).
The timer is a very important part of the atmosphere of the game, and an important balancing factor for the Genestealers.
The base time for any game is 2:00 minutes, with + :30 seconds for EACH sergeant & captain in play (Captains also add 2 CP each), and + :30 seconds for having ANY psykers in play (not each psyker).
Of course, these are 1st. Edition rules (the only rules worth playing, with the addition of Guard stancefrom 3rd.)
Last point:
Ditch the sand timer, get a stopwatch that beeps on countdown. It adds fun, suspense, and atmosphere to Space Hulk.