It depends on the complexity of the fixes. We're probably not talking additional instructions here, only some smaller microarchitectural changes (at least for Meltdown), which could conceivably take only 6-9 months to put into production after design.
I wouldn't expect anything before Q3, but Q3 or Q4 shipping does seem plausible.
The worst case is when they manage to fix Spectre/Meltdown but then introduce a bunch of other errata in the process because they rushed it. The errata lists for modern processors are already disturbingly long, and this is for things that aren't "merely" timing side-channels but actual "not as designed" defects.
I wouldn't expect anything before Q3, but Q3 or Q4 shipping does seem plausible.