I have to file my IBR paperwork in August. So this year, 8/2015, I will be sending them our tax return from 2014. In 8/2016, they will get our 2015 return, the year my husband started working and we maxed his IRA and 403. So hopefully, with the measures we took in 2015 to off-set his income, our IBR payments stay as low as possible. So, you have time to run the joint/separate scenarios at tax time. Also, my loan company has a calculator so you can predict the IBR payment based on AGI and other factors.
The addition of your income won't affect her payment until you are married and filing jointly, but the earlier you start maxing your retirement accounts, the better, anyway.