We can see that 4 new cards a day rarely jumps above 60 repetitions (except for the near term bump I have because I missed some reviews). And 8 new cards a day rarely jumps above 100 repetitions. To be safe, I think the 4 new cards a day is a fair baseline, maybe stretching to 6. This makes it ok to miss days and still be able to keep most days under 10 minutes of work.