I keep mine to 25 minutes so the thought of doing it doesn't demotivate me. Although once I get started I usually go an hour or longer!
I would say shooting for at least 25 min is good. If you can get a minimum of 25 min everyday then that's awesome. Then it would be great if you feel like you were able to keep going with your work and maybe get in 45 min or more. But I find that if the expectation is too big then it may make me feel overwhelmed and prevent me from even getting in one minute of deep work.
I'm currently writing my masters thesis and find that 25 minutes is a good start – at least you're doing something! I do however work for longer if I'm really into it and feeling inspired – what can be wrong with that?
In terms of establishing a habit/routine I think that’s enough. And also you’ll have better odds of succeeding. And I think success is an important factor. But in terms of getting things done I guess longer is better:)
I set aside a half hour for my two smallest to do items and an hour for the big one. I have it Deep Work in my rituals 3 times a day for my to do list items. If I go over, I know that I am deliberately doing so and have allowed some buffer for that. Keep in mind that your to do list should be made up of SMART tasks which means that if you have a big task you need to break it down into manageable steps. Doing so will help keep you within your allotted time.