Mine is 30 gigabytes or so. Using mbsync, it takes up to half an hour for the first sync, then 10 seconds or less to sync after that.

I do that once every minute using Cron, and it has been serving me quite well.

I don't usually open up my archive folder, when I do it takes quite a lot of time to open on neomutt at least. Notmuch works really well after indexing though.