How Many Photos Can It Handle at Once?
Practical limits for a large library, and roughly how long a big run takes.
There is no fixed limit. Libraries of a hundred thousand photos are normal for anyone who has used Google Photos for a decade, and the work is per-file, so it scales in a straight line.
Time depends more on your disk than anything else. Writing metadata is fast. Converting HEIC to JPG is much slower, because each image is decoded and re-encoded, so a run with conversion on takes considerably longer.
Takeout JSON Metadata Fixer shows progress as it goes and can be left running. Free for your first 500 photos, then a one-time $9.99 for unlimited, with no subscription and no per-photo charge.