How to setup ppp timeout on Redhat EL 5.5

I know how to setup timeout of ppp on FreeBSD as follows:
----------------------------
pptp:
	 set timeout 600
...
----------------------------

However I don't know the way on Linux, because there are no ppp.conf
file on the Redhat package.

I would like to know how to set ppp session timeout, thanx.

P.S.
I installed "ppp-2.4.4-2" and "pptpd-1.3.4-1" packages on RHEL5.5.

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