My parents can claim me as a dependant but I forgot to mark that on my return. How long before my ammended return shows that so they can file their taxes?

[DoninGA]

Your amended tax return will not change the IRS database for e-filed tax returns so your parents will have to print and mail their tax return including you a a dependent.  The IRS will pay the tax refund on your parent's tax return, if any.

It takes the IRS anywhere from 20 weeks or longer to process an amended tax return.