Does my wife need to enroll in soft power activities for the DTV visa in Thailand?

The Thai language course is not considered a form of soft power.

Firstly Thai language doesn’t usually qualify for a soft power activity (there has been an unverified mention that someone got a dtv for it but that was in the very early days of the visa).

Only the main applicant has to enrol in the activity. Dependents apply after the applicant has their visa.

How long depends on the activity and/or embassy applying. Most suggest applying for an activity that will last at least 6 months or longer.

There are many reports of people who have applied with a course of less than that and being issued a multiple entry tourist visa instead.

Some have gotten a dtv with a single dentist or medical appointment but again that was probably in the early days of the dtv.

Look at the Thai Embassy website serving the country you will be applying from to see their specific requirements as they approve your eVisa application. You get yours first then your wife applies as your dependent (or vice versa).