Regardless of placements or taking a long break, your hidden matchmaking rating which dictates your SR, never changes (even if there is decay). As such when you return, it will first pair you against players of a similar skill rating to what your skill rating was before the decay. Now depending on how much time you have been away, the game could possibly increase the amount how much your SR goes up and down based on the results of your matches, because the less you play the less certain the game is sure of your actual current skill.
More details can be found in this handy guide: