Is it realistic to expect to earn 100 onions in a week or two using the onion forum?
Yes of course as long as you're work is great and get selected for the several constest per week. VIP Dominators I think earn more than 100 so you can try applying for that too. But if you can't, the only way we can earn that much is if you own more than 2000 onions and join the Friday Airdrop.
Check out a lot of contests and giveaways and be active on the forum and this can be achieved. Not saying it will be simple, but with consistency and diligence, it is possible.