Per the IRS:
However, taxpayers would have to file an amended return if they did not originally claim the EITC or other credits, but now are eligible because the [unemployment comp] exclusion changed their income. These taxpayers may want to review their state tax returns as well.
IRS to recalculate taxes on unemployment benefits; refunds to start in May | Internal Revenue Servic...
As far as I know, they are treating these (EITC) the same as any other 1040-X amended return. That is, plan on 16 weeks. Also see:
Amended Return Frequently Asked Questions | Internal Revenue Service (irs.gov)
