B1 German is about 1 year of intensive studying from zero. With immersion and part-time commitment, I'd say ~3 years is a comfortable timeline to learn B1 German.

I am basing this off my personal experience of going from A1 -> A2 -> half-way through B1 (I dropped after I decided against studying in Germany, but my classmates continued the course). Given that German companies are known for excellent work-life balance, there should be enough spare time to learn German by the 5 year point.

All that being said, I imagine it's harder to learn a language when you have kids and family responsibilities.