You're quoting the length of Course V Maintenance in the CALGB 10403 study (or similar pediatric COG studies on which that was based), which is misleading in that it is (a) applicable only to ALL (b) patients under 40 y (c) only the final of five courses of treatment.
That being said, you're directionally correct and certainly, shorter duration immunotherapies represent a sea change.
The treatment itself is a one-day procedure (although the patients will be generally kept under observation for CRS for a few days and then regularly monitored for response), the 3 weeks is the time needed for manufacturing/QC. In that time patients receive bridging chemotherapy until the CAR-T product is ready and unfortunately some do not make it.
That being said, you're directionally correct and certainly, shorter duration immunotherapies represent a sea change.