Hey Zenon,
nice start here. I'll try to give some answers to your questions.
1) That's a very vague question, comparable to "How long does a marriage last?"From some hours to years everything's possible. The longest I spent on a map is since 2012 and still not near completion, however, I only work on it very irregularly. I guess I spent some hundred hours of work on it, probably 500, definitely 300+. Shortest map was maybe 2-3 hours.
2) I do like the tree texture. I'm a total noob on this style of mapping (it's awesome but far beyond my skills) though I think you could keep it.
3) Scales are always difficult to mee, too. As I looked at the map I thought this area could be about 500m in length. As I read you plan the river sketched in to be some hundred metres wide I was bit surprised. I'd say the map could very well be 1-3km each direction and still look fitting. So there's good space for the town.
Concerning the river wide I guess it doesn't really need to be that wide (half mile). Guessing you're referring to the US miles half a mile would mean about 800m wide. That's a truly tremendous amount of water. Think how long it took humanity to be able to build a bridge across such a length. For comparison I measured some bridge lenghts using Google maps:
The Deutzer bridge across the Rhine in Cologne is about 350m long. The bridges across the Thames in London (Tower, Westminster and other famous ones) are about 250m wide. Bridges across the Seine in Paris and across the Tiber in Rome are 100-150m wide. At the widest point where the Ile de la Cite lies, the Seine is about 400m from bank to bank right across the island.