Mirek
You can add a custom field on the Ticket Details Form (or a custom form) and add your time spent there. Of course, you will have to manually calculate the time.
We do not have any core plugins that offer "Time Spent" functionality nor have this on our roadmap at this time. If anything this is a "Feature Request" for after the v2.0 release. We do not have any set release dates for v2.0 and it will be some time before it's released.
Cheers.