That's a good question for your cardiologist. I'd think especially if he/she gave you a second rate control drug ( diltiazem) as a pill in the pocket to control the tachycardia when it doesn't stop on its own, you might need some additional instructions as to how long to expect that medication to stop the tachycardia, and what to do when it doesn't seem to be working ( ie, 1 hr, 2 hrs, etc??)
Wondering how long to wait till you think it's time to head to the ER is a valid question, I'd think your cardiologist should give you an idea when that time should come. I've read guidelines ( also mentioned in this thread), that if your heart rate is very high ( mine always got into the 190s-200's during my early SVT days), is ongoing at those rates, and/or you are symptomatic, ie, lightheaded, dizzy, short of breath, or with chest pain, it's time to go to the ER.

I have been on 150 mg/day metoprolol for a number of years, and it's done well at controlling my SVT. The bad news now is that A-fib has also reared its ugly head and I have short episodes of SVT and/or A-fib. My cardiologist has instructed me to take an extra 25 to 50 mg of metoprolol when those episodes last longer. Fortunately I have not had to do that.