I wouldn´t say that there a universally accepted time limit for postdoctoral research or contracts. At least in Europe, in some institutions or national programmes, there is a split between junior and senior postdoc, being the second one available for those who finished their PhD ten years or more ago.

Thanks Ruben for your reply, you're quite right, in some institutions the time is limited and almost fixed maybe for financial reasons.

it also depends on the the field of research.

I agree with Rubén , there are two types of postdocral study: for juniors and seniors. The first type takes longer time, while the second type takes shorter time.

I also agree with Rubén.

I think two years