Let's imagine that I have 20-30 links to directories, experience in submitting there, site title and site description, site URL and working site. I'm a typical internet-user

How much time it would take to submit one site to those 20-30 directories (manually)?

I fyou are going to copy and paste the same info into each direcorty submission page, I'd say roughly 5 minutes per directory. So, figure an hour, hour & twenty minutes overall.

If you are going to write (and proof-read) each directory submission individually - say to vary the description and/or title a bit between links - then I'd say closer to 10 minutes per directory.

I'd say more time than that. Each directory is unique and it often takes time to identify the best category it should be placed in. Sinces category hiearchy varies for each directory this takes time. Some have cumbersome multi step signup processes (msn biz directory) others are rather quick and easy. Some also require confirmation. Then you have to confirm its inclusion. One or two out of 20-30 may get bounced back by an editor for follow up.

Assuming you have the list of directories ready to go (that's work in itself) ....

1 hour prep, 8 hours working on inclusion and an additional 2 hours of follow up later on.

Especially if you've never submitted to the directory before and the process is unknown. If you've done it time and again, it'll be obviously quicker.