From a bit of reading I just did - apparently it will work in a few hours, it is not the immediate effect type (so if you are having a panic attack you may need something different).
There was also this information about taking it.
Taking this Medication
Clonazepam comes as a tablet to take by mouth. It usually is taken three times a day and may be taken with or without food. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take clonazepam exactly as directed. Clonazepam can be habit-forming. Do not take a larger dose, take it more often, or for a longer time than your doctor tells you to. Tolerance may develop with long-term or excessive use, making the drug less effective. This medication must be taken regularly to be effective. Do not skip doses even if you feel that you do not need them. Do not take clonazepam for more than 4 months or stop taking this medication without talking to your doctor. Stopping the drug suddenly can worsen your condition and cause withdrawal symptoms (anxiousness, sleeplessness, and irritability). Your doctor probably will decrease your dose gradually.
(I did a second answer as I couldn't paste that info into the comments).
Good luck