Thank you for your query at DoctorSpring.com
So you've been prescribed 10mg Inderal, twice daily for sinus tachycardia. Generally, that means you take the two tablets 12 hours apart. The only thing to be careful of with Inderal is to keep a gap of minimum 4 hours between doses else your heart rate will drop to dangerously low levels.
Additionally, it appears that you have been put on the lowest dosage possible for your condition. Physicians often do this with medications, start at a low dose and increase steadily to find out what dosage is apprpriate to control your symptoms.
I would suggest that you visit your physician and inform him regarding the quick wearing off of the medication. In most likelihood, he would increase the dosage to one which better control your symptoms.
Hope this helps,