I booked Marriott Marquis for my free stopover, I contacted them to ask if I can check in early as my flight arrives at 6AM. I am sharing their reply:
" Greetings from Marriott Marquis City Center Doha!

I have received your request via Facebook in regards to your upcoming stay with us. Please note that as part of the Qatar Airways Stopover Program, guests are able to check-in any time of the day and are entitled to stay in the hotel for up to 24hrs. I have as well attached your Marriott Rewards Membership Number to the booking.

Looking forward to welcome you soon and please do let me know if there is anything else I can be of assistance with."