You asked how long to not react for? Well it's v hard so I have a cunning way of not freaking out when DS has had nothing but a piece of toast and 5 bananas that day....
With DS we alternate meals he will eat with normal family meals. It means we can put food in front of him and be relaxed about whether he eats or not because I know that that day he'll be getting some calories. So, for your DS; yoghurt & fruit for breakfast, then normal lunch, and normal dinner. Only give him fruit for snacks. The next day you could give him milk, crisps & dry bread for lunch if it will make you feel better. Don't say anything about it though- that's just what is on the menu that day. Decide what you will offer him each day & stick to it.
Impossible although it may sound, nonchalance is key. The minute we stopped bargaining with DS his eating improved. We do occassionally get him to 'kiss' or lick foods he won't normally eat but never ask him to eat it. Occassionally now he will put a bit in his mouth then immediately spit it out but he gets lots of praise for trying.
He never gets praise for finishing a meal or eating lots of something that's good for him. He's too little to learn about nutrition, we're just trying to help learn him not be afraid of trying new things.
We also don't refuse him dessert if he hasn't eaten his mains. Whether it is fruit, yoghurt or ice cream he gets some dessert regardless of whether or not he's eaten anything else. So there's no labelling one sort of food as boring and must be eaten and another as treat food. It's all just food.
It's really hard some days not to despair but having also been that fussy child, and remembering dreading dinner and those nightly battles, I know that this is the right way for us.
Hope you find the right way forward soon. Remember you are not alone though, it's almost a toddler rite of passage!