In your example, you should declare minimum fuel as soon as you commit. I might also point out that a full hold does not take 4 minutes but rather 5.5 - 6 minutes, so you have 3 holds. It appears that you will have about 9 minutes of fuel to spare and that means if they give you even 1 more hold you might infringe your minimum reserve.
It helps to look at various scanarios. Instead of numbers, let's look at time. Let's say you check in with approach and they hand you an EAT of 16h00 and this requires you to commit. You work out that whilst you have to commit, you must leave the hold by 17h00 in order to land with FR intact. In this case I am not worried about minumum fuel. If. however, I was handed an EAT of 16h55, I would be worried. If my EAT was 16h50, I would still be worried, but a little less worried. If my EAT was 16h45 again I would be concerned by probably not overly so. The question we have to aski ourselves, is at what point do you go from being "concerned" to actually doing something about it - in a modern environment with good ATC that I know, I would probably be happy with a 10 minute margin. Unfamiliar turf(or 3rd world) and that margin might go up to 15 or 20 minutes. Not everyone will agree with where this margin sits and it is up to you to think about where your limits lie. Once you have an EAT and you have calculated the margin falls below your comfort level ie you have "calculated that any change to your clearance might infringe your final reserve" then you may legally declare minumum fuel as soon as you commit.
The act of declaring minimum fuel does not require any action on behalf of the ATC but they will ensure that they stick to the EAT that they gave you. I might also point out that when they are trying to address what may be a messy radar screen, so they may move you around in the sequence and knowing that you don't have much fuel beyond your EAT is something they should be told sooner rather than later. I always say that when it comes to fuel, don't be a hero.