I have a KOTAK Mahindra ULIP Plan which has life insurance cover also. I am paying an annual premium of 30000 which is due till 20 years.

Is it worth keeping the policy and paying premium.

Or if I stop paying premium and keep my money intact with Kotak Mahindra, when should I withdraw it?

First of all the private insurance company's are not reliable.you cannot trust on them.LIC is better then these company's.the unit linked plan depends on share market terms and in present share market sucks.so don't invest in unit linked planes.
as a long term investment it is a good plan because future of share market will be good.But i have not trust in private company's.so i suggest LIC .

Ulip policies are considered best suited for long term as first 3-5 years the expenses are very high (20%) and after 5th year onwards charge 4-5%, so if the fund you selected performing well and given good return then one can keep holding for long term as ulip plans are actually not for 3-5 years as marketed(sold)

You need to ask this question
Why you invested in Kotak Ulip Plan
For Insurance
For Investment
For Tax benefit


If answer is for Insurance Purpose then i feel pure term plan without any return will be best suited as will have less premium amount with good Life cover and if you invest remaining amount in mutual fund will give you better return.

Why one should select Term Plan + mutual Funds??
http://mutual-funds-personalfin.blogspot.in/2011/07/why-one-should-select-term-plan-mutual.html


If your answer is for Investment or for tax benefit

I feel in both the cases better investment options are available for tax benefit as well as investment.

I don't know you been investing how long in ulip plan, but if you completed 5 yrs, (if you are not getting good return till now from that ulip plan) then you can stop and redeem full amount or other way is converting it to paid up policy by visiting Insurance office and submitting relevant document as don't want to close this policy, but don't want to pay more premium
check this:
Turn your insurance policies into paid-up plans (If you are thinking of discontinuing an insurance policy or want to stop paying the premiums)
http://mutual-funds-personalfin.blogspot.in/2011/07/turn-your-insurance-policies-into-paid.html