How fast does basal cell skin cancer spread if the doctors report came back "deep"?

Very very slowly: While basal cell carcinoma can metastasize and spread from the skin, it is very unlikely to do so unless they are very big (larger than the size of your fist). For the most part, bcc are slow growing cancers that can destroy surrounding tissue if neglected. So if you have been diagnosed with one, i would followup with your dermatologist or mohs surgeon soon to get it treated.

Difficult to predict: There are fast and slowing growing skin cancers. Basal cell in general is slow growing and spreads locally rather than metastasizing (spreading thru the blood stream or lymphatics to other parts of the body) so usually not fatal. It can be very locally destructive so needs to be taken care of in a timely fashion. There are some more invasive types of basal cell such as infiltrative or sclerotic.

Report: Basal cell carcinoma of the skin is usually very local, meaning it doesn't often spread, even though it can recur. If the tumor is present at the tissue edge or margin, this has a high risk of recurrence, and usually needs to be re-excised for clear margins. People with basal cell carcinoma are generally at risk for getting these other places and should be examined yearly by a dermatologist.

Dr Friedlander and another doctor agreed

Need further removal: If you had a positive margin (i.e. There is still some basal cell cancer left behind) when your surgeon removed your skin cancer, you probably need more surgery. Basal cell cancers are generally curable with excision. If you still have some cancer there, it is much better to take care of it now than wait until it spreads enough to be come visible again. A moh's surgeon is probably best choice.