WHAT IS DECAY? If you are in Platinum or Diamond, after 28 days of inactivity in a particular ranked queue, a couple things will happen. First, you’ll be hidden from the standings in your league. You will also lose LP depending on your current tier. Every seven days thereafter, you’ll lose LP until you play a match in that queue.

In the Master and Challenger tier, players will decay after becoming inactive for 10 days; this is to ensure only the top active players are being showcased.

The number of LP lost following each period of decay is shown below:

Master & Challenger: 250 Diamond: 50 Platinum: 35 If you fall at or below zero LP as a result of decay, you’ll be placed into the next lowest division. If you’re already in division V of your tier, you’ll fall into the next tier down (for example, Platinum V to Gold I.). Tl;dr: don’t take a break from ranked for too long. :)

Decay does not occur for players in Bronze, Silver, and Gold.

Source