Adjustable dumbbells to 24 kg and a bench in a spare room - how far can that actually take me?

It caps out much later than you think if you stop trying to replicate barbell movements and start using the tools that make a light weight heavy.

Unilateral first. A rear foot elevated split squat with 24s in each hand is a genuinely hard set for most people who can back squat a bar. Then single leg RDLs, step-ups to a real height, and B-stance hip thrusts on the bench.

Then the variables that are not load: tempo with a three second lowering phase, pauses at the bottom, one and a half reps, and shortening rest periods. A ten rep set at a normal tempo and a ten rep set with three second eccentrics and a two second pause are not the same set.

My hinge stalled first, which is the honest weak point of dumbbell training, and I fixed it with single leg work and a band rather than by finding heavier dumbbells.

Fourteen months of home training with a similar setup, so here is what actually happened rather than the theory.

Upper body: never a problem. Pressing and rowing progressed the whole time and I was nowhere near topping out the dumbbells.
Lower body: progressed for about seven months on unilateral work and tempo, then genuinely stalled. Split squats at 24s for sets of twelve is a conditioning session, not a strength session.
What I added at that point: a weighted vest for step-ups and split squats, and heavy carries. That bought me another six months.

So not three months. But there is a wall out there and it is a lower body wall, and you will meet it.

Practical disagreement with making 24 kg work indefinitely. If you were already comfortable with a barbell, a basic bar and a couple of hundred kilos of second hand plates often costs less than another set of adjustable dumbbells, and used plates turn up constantly. You need about two square metres and a floor you do not mind. If you are in a first floor flat, ignore me entirely, but if you are on a ground floor or have a garage, buying load is cheaper than engineering around not having it.

My mistake was chasing novelty. Every week I found a new exercise on the internet because it looked like progress, so nothing was ever repeated long enough to add weight or reps to. Five movements, the same five, for twelve weeks, writing down every set. Boring, and the only period in three years where my numbers moved consistently.

One space note since it is a spare room: get a proper rubber mat or two under the working area before you drop anything, and check that the bench height works for your pull-up bar height if you are using it to get up there. Small things, but they are what makes the difference between training three times a week and training when you feel like moving furniture.

Intensity techniques before more weight. Slow eccentrics, pauses, one and a half reps, and taking sets genuinely close to failure. Most people at home are training three or four reps short of where the stimulus is.