Sorry to hear about that, but lymphoma is very treatable and I know two pilots who have a history of it.

As for how long it takes to get your medical back...it depends entirely on your unique circumstances. You cannot rush the legit medical issues involved, but you might be able to preemptively avoid any bureaucratic snafus. You should pay pay an aviation medical consultant to review the process and help you avoid unnecessary delays.

As far as pilots working while on chemo...I suspect that was a case where the chemo was being done as a purely precautionary measure and there was no evidence of metastis, ie a lone skin cancer was removed with no sign that it had spread.

In the case of an internal tumor such as a lymphoma, they may need to see some evidence that it is eliminated from your system which might involve waiting. But I'm not an expert.