According to rxlist.com:
A single dose of 8 mg of tizanidine reduces muscle tone in patients with spasticity for a period of several hours. The effect peaks at approximately 1 to 2 hours and dissipates between 3 to 6 hours.
So if you take a 24 hour day and do a little math magic you can take up to 4 8 mg pills in a day. Jenny, you are just fine.
By the way I am a strong advocate for Robaxin. It has been the only muscle relaxant strong enough besides Soma (which doctors are trying to stay away from because of its addictive issues) to help me.