With the upcoming Android 5.0 KLP release due soon after Google I/O in May, for the Nexus line, good luck getting a 5.0 ROM anytime soon for the S4. With a fully new point release, new ROM's based off a big release like that take extra long to release on non Nexus devices.
CM11 for non Nexus phones will take months to come out, and be rough alphas to start, sure glad I have the Nexus 4 now.
Assuming everything happens linearly, the same way with the same timing as it has in the past is presumptuous at best. With every major new version they have been tightening up and streamlining these relationships and update delays between google, manufacturers, carriers and end users.
As Android continues to eat away iOS marketshare I'd also anticipate Google slowly taking the free reign that the carriers enjoy now back out of their hands, starting with a checkbox to receive updates direct from Google. Just a hunch.