What is the update timeline of iMac?

We are all just fellow users here and have no more information than you do, and Apple does not provide release information until such time as they are ready to make a public announcement at which time we'll all find out together.

Regards.

Apple just recently updated the iMac line so don’t expect anything in the short term. What specifically are you looking for that is not currently available, there may be other solutions you have not considered.