Studies have found that it's enough to have 11 minutes per week of cumulative exposure to cold water where cold means anything below 18°C. So you'll reap the same benefits of spending 30 min per week in 5°C water as if you would spend 11 min/week at 17-18°C.

And it doesn't have to be 11 min in one go. You can spread it apart in smaller chunks every day 1-2 min per day.