The medicine is taken every day. There is no "rest period". The lab is required to evaluate you each 21 days and with acceptable blood tests, etc will issue the next 21 day dose. Scans are about every 6 weeks.
There are many cohorts now for Revolution Medicine KRAS drugs. Monotherapy 6236 (the panKRAS drug) is in phase 3, available to pcan patients but also colon cancer and perhaps other types. The drugs have now been named as well. I have had small progression now, and my CA19-9 has risen from normal range to 145. Next scans will tell whether I am allowed to stay on the drugs or will need to look elsewhere and possibly resort to targetted radiation - which I have been avoiding.