how to control the time needed to hydrate/dehydrate a process instance?

Dears

items in work list need to be handled in a reliably short time. this time varies from firm to firm; in a firm that handles 15000 items daily, dehydration time should be less than a firm that handles 500 or less items daily.

i thought that this should be configurable due to the differences in firms conditions.

Your question is analogous to asking "how long would this web page take to submit a transaction?".  In short, there isn't a straight answer as this would depend on your application design.

In the K2 context of the process instance, this would depend on the amount of data stored in the process instance and also the type of hardware you are running on.

There are also other factors like Data-on-Demand that will also impact the performance of the worklist and when opening work items (The K2 best practices guide available on the KB site should summarize most of these tips).

 However, you need to run your application on properly configured hardware and with proper load to get realistic figures.