How long "Annexed subject" diplomatic penalty stays after annexing vassal?
As long as I can remember (I started out in 1.7 I think), the following has been true:
Annexing a vassal will result in a 10 year penalty of -3 diplomatic reputation [DR] to the overlord. The penalty does not stack if multiple vassals are annexed. All other vassals of the overlord will have their opinion reduced by -30 as well, which will stack. If the annexed vassal was a member of the HRE, then the opinion penalty will also be extended to all members of the HRE. source
Most important is that the effect will
- delay or freeze other annexations (if DR gets less than 1, even during an annexation)
- potentially prevent starting another annexation (if attitude of the other subjects gets below 190 due to the malus)
The latter stacks, so by annexing 3 subjects, the others will all gain -90 rep, which is very considerable.
A good strategy is to keep you DR positive (i.e. +4 before the first annexation), if possible, and annex them in increasing order of attitude (the most negative to you subjects first, so the penalty only affects those subjects which are still at, or close to, 190+). Note that this requires some careful planning.
Important: If you cannot keep positive DR, annex only one of them at a time, and wait 10 years inbetween. If you don't, a diplomat will be stuck for 10 years.
If you stay at positive DR, you can also try to simply annex all subjects simultanously, so the -30 penalty won't affect them (it does not matter once annexation started).
Also note that the -30 malus is applied to exactly those subjects you have when an annexation finishes (not starts). So you should wait to create new vassals until after all or most annexations have finished.
According to wiki the penalty lasts 10 years.