I disagree. I highly recommend a timer, but rather than an alarm, use something like a quiet bell. I use the Insight Timer app, but just without all the social aspects that come with it.

Sitting as long as you feel like is not beneficial, because most of the actual work will happen when you start to notice your agitation and at that point is when you will just want to get off the cushion. Stick through it, and watch that agitation and your monkey mind go crazy, until the end of the timer.

I recommend starting with a short timer, 10 minutes, then slowly increase the time.