I budgeted ten hours a week for the side thing and it has taken over the whole week - how do people contain it?

You are right that the hours were never the constraint. The thing that expands is attention, and it expands because the work has no boundary telling it when it is not happening.

What actually contains it:

Fixed slots, same times every week, and nothing outside them. Not ten hours somewhere in the week: Tuesday and Thursday evenings and Saturday morning, specifically. A floating budget is spent continuously; a scheduled one has edges.

Notifications off, entirely, outside those slots. This is the single biggest change and the one people resist. If a customer email can reach you at work, the business is running twenty-four hours a day in your head regardless of when you do the work.

One capture place. When something occurs to you at eleven at night, write it down and go to bed. The eleven o'clock work is almost never urgent; it is the discomfort of holding a thought. Writing it down discharges that.

A stated response time, on your site and in your replies. Within two working days is completely normal for a small business and it removes the sense that everything must be answered now.

An end time on the Saturday slot, with something after it that you cannot move. The morning becomes the whole day because nothing stops it.

The structural half, which people skip because it feels like a business decision rather than a time one: what you sell decides how much it can expand.

Some offers are unbounded by nature. Anything custom, anything ongoing, anything where the customer sets the scope. Those will eat whatever time exists, and no amount of scheduling fixes it, because the demand genuinely is larger than your slot.

Some offers are naturally bounded. One narrow thing, defined deliverable, fixed scope, delivered and finished.

If you are in the first category alongside a job, the honest options are to narrow what you sell, put fewer customers on it, or raise the price so fewer people buy and each one is worth the disruption.

Two specific things that helped me:

Say no to the interesting exception. The custom request that is slightly outside what you offer is the one that consumes three weekends. It is also always the most tempting.

Batch the admin. Invoicing, email, bookkeeping in one slot rather than scattered. Scattered admin is what makes ten hours of work feel like thirty.

And be honest about which category you are in before you try harder at scheduling. Discipline cannot fix an offer that has no edges.

The unpopular thing worth saying plainly: the day job is funding this and it goes first.

Not for moral reasons, for practical ones. The salary is what makes the side thing survivable, lets you say no to bad customers, and means you are not pricing out of desperation. Damage it and you lose the conditions that make the experiment possible at all.

So the boundary that matters most is the one at work. Phone away, notifications off, no checking between meetings. If something genuinely cannot wait until the evening, that is a signal about the offer rather than a reason to answer it at your desk.

On the partner, since you mentioned the deal: have the conversation again now, not when it becomes an argument. Say what has happened, that you noticed, and what you are changing. The version where this goes badly is the one where the creep is never acknowledged and resentment builds on both sides.

And set a review point, three months out, look at whether it is earning enough to justify what it costs, and be genuinely willing to stop or shrink it. A side thing that is quietly eating the week and not earning is a decision that has never been made rather than one that was made and is being honoured.