The exact time of numbness is actually unknown. It all depends on the patient's tolerance to pain and the patient's physical health conditions. Some patients experience numbness within seconds or even minutes. Other patients experience numbness within 15 to 20 minutes. And sometimes there is no numbness at all which would require a second dose of the numbing agent known as novocaine. If, at all, you can actually feel the drilling, you need to tell your dentist because, again, this means that the novocaine didn't do it's job. It is very painful and uncomfortable if the novocain isn't doing it's job.
I can tell you that the last time I saw a dentist and needed the novocaine, it didn't work and had to have a second dose. It hurt.