[Dovecot] rule of thumb for indexing overhead

[WJCarpenter]

I realize it's hard to be precise about this, but does anyone have a
feel or rule of thumb for a couple of aspects of indexing overhead?

1.  Proportionally, how much space does it take for all 4 files?  If I
want to give my users a quota of 100 MB for messages, how much real
space should I plan for so that I won't run out of space for indexing?

2.  What's the overhead in rebuilding index files?  Suppose I use an
LDA other than dovecot, so at least the INBOX index is frequently
getting out of date.  Does it cost much (in CPU, memory, and disk IO)
to rebuild the index files?  (I'm using maildir.)