Render range is 40km, you'll never see/hear anything beyond that. Ships will pop into or out of existence when they pass that range.
Hydrophone range is generally around 30-35km for initial contact from what I've seen.
The starter hydrophone works on the surface and this means you'll have approximately 1000% more fun since you'll catch a lot more contacts. Once you upgrade your hydrophones, or use a mod like UHS which correctly stops surface hydrophone contacts, you'll be lucky to find a ship, ever. You'll have to spend the time ducking under the surface every 2 hours, hoping you're close enough to something to hear it. At first it's "realistic" and "fun", then it's "tedious" and "stupid".
I usually end up meeting enough targets or a convoy on the way to a patrol zone so that I sometimes don't make it there at all. Otherwise I just poke around known shipping routes for targets.