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?

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.