Q: Exact/effective start date of migration
A: The exact/effective start date can get delayed in special cases
Lets say you time a initial migration for 01.06.2021 13:22.
Now depending on the load of the source server or our migration infrastructure, it is possible that the migration cannot be started at exactly 13:22:00, but get delayed for a minute or two.
We do not think that this is a major issue, as the migration itself might take (depending on the size of your mailbox) up to hours, or even days.
We have done literally thousand of manual migrations (with a kind of manual precursor of this tool), some for testing, some for ourselves and above all, many many for our customers. From our experience, we can say that migration duration depends heavily on factors beyond our control (source/target server, size, speed etc etc).
If you want some numbers:
The largest mailbox in terms of "number of email messages" we migrated was about 600000 messages in one single mailbox (in under 1 GB). This special migration lasted for about 48 hours due to huge number of single messages and slow source server, but the average message size was very small.
The largest mailbox in terms of "data size" we migrated was about 180 GB in one single mailbox. This special migration lasted for about 8 - 9 hours.
As you can see from these 2 examples, a 'small' migration with under 1 GB can last substantially longer than a 'large' migration with 180 GB.
Another special case can be if you time many migrations for the same minute, like e.g. 10 or 20 migrations are timed for 01.06.2021 19:00
In such a case, it is better to split up the starting times, like e.g. 3 for 19:00, 3 for 19:01 etc