Well my first channel took 3 months to develop, working on it with as little sleep as I could get away with 7 days a week. I could now write a channel that does what you describe in a few days. Since you have experience with other languages, let's say conservative estimate for first time developer would be 30 working days if that is all you are doing, add two weeks to that and there's your estimate if you devote all working hours in that period to the project.
Or, hire a roku developer (like me) to do it for you.
What you need to figure out:
1. Get list of videos from server in XML format
2. Parse that and store in an array
3. Create a posterscreen
4. Load array into posterscreen
5. Handle user input to posterscreen
6. Display info in a springboard screen
7. Load a video from the array into the videoplayer
8. Handle end of video condition
So that is a roadmap of 8 functions to build, function 1 calls 2, 2 calls 3, etc.
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