The gemara Brachos (35a) says that it is forbidden to eat without making a bracha. The gemara adds that it is stealing from Hashem. This implies than more than an obligation to make a bracha, under no circumstances may you eat without making one. If it is a mitzva to make a bracha, then under duress when you are an Ones you may eat without one. But if it is forbidden, then only in situations where it is Pikuach Nefesh, life dependent, would you be able to eat without making a bracha.There are times and places where you may not make a bracha. For example it is forbidden to make a bracha if there is a Jewish married women who has her hair uncovered, or a women is not dressed properly and you cannot turn your face in another direction. Most of these situations can be cleared up and you can then eat, but what if it lingers for a long time. For example if someone is in a hospital where there is a bad smell or the person himself is unclean, must he wait until he is on the threshold of death before eating?
Rav Vosner (Shevet HaLevi 6:23) says that it is clear to him that in these situations you may eat without a bracha. He brings proof from the Mishna in Brachos that says that a Ba’al Keri should not make a bracha before he eats. The Mishna in that case is surely not talking about a case of Pikuach Nefesh, says Rav Vosner. Therefore he says that your hunger need not be life threatening, but even if you are an Ones because you would be forced to go a prolonged period without being able to make a bracha, you may eat without one.
How long is a prolonged period? Rav Vosner says a full day just like a fast day. This is considered painful and damaging and would qualify you as an ones.