If I would go on low dose conventional hormone therapy ( an estrogen and a blocker ) but have no intention to socially transition at this moment in time , how long could I delay it ?

I'm in college for the next 2,5 years and thus not keen on coming out . I also still live at home , although I'm almost  20 , where my gender dysphoria is a taboo and is not welcomed to be discussed . However my dysphoria is so badly that I need to start hormone therapy as soon as possible.

How long can you generally delay social transition while on a low but working dose of hormones? Also,how strong is the possibility that your environment begins to notice changes?

A lot of it depends on the reaction your body has to the HRT, regardless of dosage. Some people progress quicker than others. Unless you start growing C, D or DDs (highly unlikely) then you won't have much of a problem in all but the lightest or tightest clothing. A little bit of creative layering or loose clothing should see you good for two or more years. HRT feminises you, but it isn't the be all and end all in transition - that also requires changes to clothing and presentation. If you're not including those most people won't notice a thing unless you go swimming or wear a tight tee.

As per what Grace says.

I have been on low does for over 2 years and it really has helped me.
Yes I have two squishy bits that can be hidden.

You will be very surprised even on low dose as to what E can do.

I did it 5 years or so, but eventually got really depressed and switched over to a full transitioning level of HRT. It was great for a while.

If low 'Low' works, does what you need it to do like lessen the GD or the depression or anxiety then maybe it will be not quite as low 'Low'. But, just exactly are you goals? ALL the goals?  For instance, I've been on/off low dose E or E with an AA several times over the decades for the much needed Brain Reset. The Prime Directive for me at the time was just to be 'Normal', or as best as I can get to it. If I was not currently in I soon was able to be in a relationship with a woman. For me though it was in my late teens to 30's, AKA peak T, it was inevitable that things eventually started taking a hit below the belt  . AKA in very serious conflict with The Prime Directive, to which there was only one viable solution, stop. Which was OK since I had achieved the reset. This was good for years depending how deeply into excrement my life had gotten.

Several years ago I once again needed to start. Low soon went to full. I know all too painfully well what happens if my E drops or T rises  I've been living as male with a small B cup for about 4 years now. Occasionally my wife may 'suggest'  I change my top because of my boobs. But since it is very likely to be something that was OK it is her hypersensitivity. You really got to looking, and knowing, to see. Being a former fatty I never ever wore (in male mode) tighter fitting clothes. Even in say a tee shirt bra the girls can go unnoticed.

As with all things HRT, YMMV. Low dose may do squat for the GD. It may also knock you on your (emotional) but. There may be no onset of female secondary sex characteristics, or you can sprout a B cup almost overnight. Your libido can be that of a 16 yo high school male, or go totally into the crapper after a week.

I started on a very, very low dose of E only at age 32 and was able to hide changes for ~3 months before people noticing forced my hand.  I may be unusual, though.

The blockers will help reduce the uncomfortable feelings you have. How much depends on a person. Estrogen tends not do to much for you emotionally unless you desire a more feminine body. I was estrogen only and a pretty strong dose at that and because the T wasn't shut down, the uncomfortable was still pretty strong.

Children are given blockers to prevent puberty from altering their body and they may remain on them from age 13 to 16 or 18 depending on the treatment plan so there shouldn't be a problem with you remaining on them until you finish school. Once you are near the end of school or out of school you can reevaluate your status and if it's time, add estrogen into the mix.