If the MS receives a paging request for itself and is allowed to access
the network, than it will start the immediate assign procedure (The details
are in GSM 04.08, a good overview with reference to the specification is at
http://en.wikipedia.org/wiki/Um_Interface, many thanks to David Burgess for
writing it down). The MS will not react on further paging request as long as
the immediate assign procedure is running.

The BSC can't be sure that a single paging request is received by the
MS so it (optionally) has to repeat it (there can be distortions, weak
signal and so on).

> Normally the RACH is sent 3 time slots after the paging request right?

I don't think so, the RACH is always on TS0 of the uplink (at least this
is how I understand it) and the PCH is not necessarily on TS0 (depending
on the configuration). The reaction of the MS surley depends on the
firmware/hardware of the phone and how fast it works, so you don't
have a fixed delay. The MS usually has enough time to react, the
T3113 timer defines how long to wait for a PAGING RESPONSE and than
optionally repeat the paging request (if and how often the paging
request is repeated, is not defined in the specification). I don't
have exact numbers for the T3113 timer because its up to network,
but common values seems to be around 5 seconds.