Two numbers matter: new cards added per day and total reviews per day. A balanced ratio is around 20 new cards to 100 reviews — enough new material to keep progressing, not so much that your review queue explodes.
If your daily queue takes longer than 30 minutes, you're adding too many new cards. Drop the new-card rate to 10 and let the queue stabilize for a week.
For exam crunch time (USMLE dedicated, MCAT month-of), 300–500 reviews per day for 6–8 weeks is normal and tolerable thanks to the algorithm doing the scheduling.