In order to qualify for Silver membership you need 10 nights credit each year, Gold is 50 and Platinum 75.
Lifetime status does not award the nights, instead if you don't reach the nights for your status by yearend lifetime kicks in and compliments you the status for the next year. Hence lifetime Gold does not effect the number of nights required to acheive Platinum.
I now have 2.1mill points and 620 nights so I'm lifetime Gold but 130 nights short of lifetime Platinum. I am currently Platinum because I had 120 nights last year and rolled over 45 to this. I've stayed 47 nights this year which means I'm at 92 nights and hence keep Platinum for 2016. But as I only stayed 47 nights this year none will rollover to 2016 and I will start 2016 at nil.
Though I've made lifetime Gold, to retain Plat for 2017 I will still need to make the 75 nights in 2016.
If you haven't made 75 nights this year you will slip down to Ggold next. In 2016 if you don't make 50 nights then due to your lifetime status Marriott will compliment you Gold for 2017. If you want to make Platinum you have to get 75 elite nights, the most common way to earn this is to stay overnight at Marriott group hotels for 75 nights, but don't forget the free credit card gives you 10 nights credit towards status, the fee-paid Chase card gives 15 and 1 extra for every $3,000 spent on it. Meetings at Marriott gain 10 elite nights credit each, even an hour one-to-one meeting!
Short answer. You need 75 elite nights credit to make Platinum, unless you're lifetime Plat. Lifetime Gold does not operate to reduce those nights.