Validity of Prescribed Meds After Prescription Expires

I am currently prescribed 20g per month, but only get through about 10-11g.

When I order 20g, I don't get through it all in a month, so at the end of the 4 week cycle I have an amount left over. I have quite a bit left over from previous prescriptions and don't need to order any more for a while.

My question is, am I still legally covered while I am working my way through the prescribed meds into the following months?

If ordered 20g at the beginning of December, and am still working my way through it in January and haven't ordered a new script, am I still legally covered?