Change tracking is near real time. So changes which occur in the tables you
are full-text indexing will appear in the index within seconds depending on
a few factors (content - XML is very slow, TIFFs are really slow), hardware,
number of changes, and the load on your SQL Server.

Change tracking can cause locking and fragmentation of your catalog. You
should run an organize periodically.

Incremental populations are almost as lengthy as full populations, use a
full-population over an incremental population whenever the majority of your
database changes at any one time.

Use an incremental population when a large number of rows change in a short
amount of time, and this large number is not a significant portion of your
total table.

You will have to study which works best for you on your hardware, your data,
and your data modification patterns.