I am attached to 2 projects: World Community Grid and Rosetta.(50%/50%)
I am always connected to the internet, and my laptop usually runs 16h/day. or even 24/7.

Which are the best settings for:
Additional work buffer
Tasks checkpoint to disk at most every
Switch between applications every

Thanks in advance!

I would suggest leaving the default options alone, if you do not feel very comfortable adjusting those options.

However, I personally set Additional Work Buffer to 0.10 days, Tasks Checkpoint To Disk to 120 secs (to reduce disk stress), and the default 60 mins for "Switch between applications every".

You may want to adjust the options according to how many/much tasks the computer can handle from these projects. If you are finding your laptop running through tasks very fast, you can increase the Work buffer value to something like 0.30 days or more, and vice versa if your laptop is sluggish in completing the tasks given.

(edit: spelling error)