There are probably better places to ask this. I mean this community is for all types of question but I think you'd have better luck asking on a more specialized community.

This 100% depends on which specialties youre talking about and how much/what experience you already have. My wife changed from ICU to endo and only took a month to transition.

So, not exactly what you're asking, but: I worked for 4 years as a psych tech before becoming a nurse, then switched to ED when I graduated. It took a solid 6 months in ED to start feeling like I had any idea what I was doing and not feel like I was in a panic every single day, then another 6 months before I started feeling actually somewhat comfortable. Part of this is because I was a new nurse, and part of it was because ED has a steep learning curve (the orientation period was 5 months iirc). I suspect it would have been an easier transition to ED if I had already had some experience as a nurse on something like a med-surg or cardiac unit. I switched back to psych as a RN and it really only took a few weeks before I was "standing on my feet" as it were, but I already had a good deal of psych experience from being a tech and I wasn't a new-grad nurse anymore.

What do you mean by dedication? As in time/effort to acquire the necessary skills/knowledge, or emotional fortitude? Definitely oncology and hospice require a lot of the latter, as you and death will become familiar company. ED sees a fair amount of death and it tends to be constant demands, constant action, with days where you wont have a chance to catch your breath. Psych can be soul-sucking at times and tends to be at higher risk of violence from patients, but there's also a lot less dying or cleaning up of blood and feces. How much dedication a specialty might take will depend in large part on what your strengths are and what you can tolerate or are willing to put up with. I hated ED because of the constant pressure and never knowing what will come through the door next, but I love psych despite often having to work with people with aggression problems and having been hit a few times.

Are you thinking of changing specialties? What's your motivation? What do you want to do now, and do you have something particular in mind? I suspect the ease of transition will mostly depend on what experience you already have and how much skill/knowledge overlap there is with the new specialty.