A course of five to ten injections are usually required, depending on the drug used. With other drugs the rear limbs can be used. This should be done 4 times daily if possible. Yes, a tortoise can stop breathing for a considerable time, but a sick animal usually hasn't the strength to do so and therefor nebulising does work in a number of cases and certainly should be tried. Finally, do not make the mistake of comparing RNS to the human common cold or flu, and think that it will clear by itself if given time.
Treatment is essential, and as soon as possible.
The longer RNS is left the more difficult it is to clear. RNS can and does kill, don't let your tortoise become a victim. Inspect nostrils daily if possible and get expert help fast if you spot any nasal discharge. If this does not work, consult your veterinarian for advice. This condition is almost invariably the result of poor dietary management.