Mining for litecoins is not very profitable.
I do not mine litecoins myself, but assuming Illumynization is earning $3 per day based on 16 hours per day and there are 30 days per month, that means he is making $90 per month. I am assuming 16 hours per day because he is mine while he is sleeping and working. When at home I assume he wants to do other things with his PC like playing games.
Of course it's not $90 for "free". You need to also pay for electricity as well. I would guess that an i7-4770k and a R9 280x uses about 200w per hour when mining . That works out to 96KWH of power per month. Assuming $0.10 per KWH that's $9.60 in electricity leaving you $80.40 per month.
Don't forget about possible wear and tear on the PC itself, specifically the CPU and GPU.