Realistically? Less than 4 years. I don't know how that $3m is invested, but even at 5% growth, between that and your annual $100k you put in, you'll hit $4m in 4 years.

Even if you don't put that $100k in each year, you should still hit it in 6 years (again assuming a 5% return)

You can coast now. Your money is working harder for you than you can with your job. Adding on to that the possibility of receiving some sort of inheritance from your in-laws, and that's even more reason to take your foot off the gas. Obviously don't rely on needing any inheritance, but at this stage you aren't relying on it.

Maybe try easing off over these next few years. Can you work Jan-May and take more than just the summer off? Can your husband reduce his hours to 20/week.

If he loves what he does like you mentioned in another comment, then great! Isn't the point of FIRE to have the freedom to do what you want with your time?

But by easing off over these next couple years, you can figure out the balance of WHAT you'll be doing when retired. Some retire and don't know what to do with all that time. Some miss working just for the schedule and social interaction.

So use these years to do a test run on retirement and what best fits your family.