。
If you play more, you'll have less time to study.
Literal
Play [subject-が] increase-[conditional-れば] study time [subject-が] decrease-[become-ことになる]-probably.
Nested consequences. The ~ば conditional gives a general rule; ~ことになる ('it will come about that,' 'it ends up being') frames the consequence as an external result, not a personal decision. Contrast with ~ことにする ('decide to'). 減る ('decrease, diminish') is intransitive; the transitive counterpart is 減らす ('to reduce').