To some extent this depends on:
Length of publication
Level of content
Number of index entries per page of text
Density of text (print size, amount of graphic material)
Additional research required to produce accurate entries
An average 200 page book written at a general level can be indexed in 7-10 days.
More complex or longer texts require more time. Longer indexes also take more time
to check and edit because of the complexity of inter-related entries:
A 400 page academic level text index would take longer to compile
and also more than twice as long to check and edit - in total around 3-4 weeks