@bihisello
havent decided duration but it will be less than a month and with a if condition that nifty PE should be less than average.
like simple algo
devide min. to max. PE in fixed intervals and assign fixed amount to it like
 PE      amount
10-12  10000rs
12-14  8000rs
14-16  6000rs
16-18  4000rs
and so on.
also investment missed in months when PE>PEavg should also be utilised when PE<PEavg.
what do u say???

http://spreadsheets.google.com/pub?key=tZ7Z5UykZoh727I7twWGO9A&single=true&gid=0&output=html

here is how PE varied from 1999-2010. i have taken data from nse website.
http://www.nseindia.com/content/indices/ind_pepbyield.htm

I had calculated mean/mode of PEs of nifty 50 index between 1999 to 2010
avg=18
mode=15

but still in VIPs case also
how i should decide at what level should i start. At preset nifty PE is 20, is it  overvalued, fairly valued or undervalued??
I want to have some metric(like PE) so that my emotions dont drive me...

The funny thing about the market Pe is that even the market participants do not know the mind of the market even though collectively they constitute the market. No wonder estimating Pe is an educated guess work rather then an offshoot of an inflexible mathematical formula.