It sounds a bit formal to me. If you want to sound more casual, you could say: I only let myself use social media for two hours a day. OR I only allow myself to use social media for two hours a day.

It's natural, yes. It could sound a little bit more natural if you added 'just' or 'only' before 'two hours'.

You can certainly say that. It is not the clearest use of "restrict". Your sentence could answer these two questions: 1. What is being restricted? Answer: your use of social media 2. What is it being restricted to? Answer: two hours a day "I restrict my social media use to two hours a day."