Unfortunately, it's really hard to generalize. If the evaluation involves Nixpkgs, for example, Nix needs to copy Nixpkgs into the Nix store, which is many MB. And of course this needs to happen for all flake inputs. I've seen this evaluation take a few seconds and I've seen it take several minutes, but in general the more flake inputs the longer you can expect it to take.
> If the evaluation involves Nixpkgs, for example, Nix needs to copy Nixpkgs into the Nix store, which is many MB.
Only if you use flakes (and this behavior is a good reason to not use them).
Using flakes or not, any store path evaluation involving Nixpkgs is going to be computationally intensive, potentially too intensive for a memory-constrained device.
(DetSys Cofounder here) I've had Raspberry Pi's give up their last ghostly breath evaluating NixOS.