Figure how far you want to/can walk each day and work backwards.
Or, you might want to consider Ponerrada, it's 200 K out and will give you the joy of climbing up and down a 930m hill as well as climbing O'Cebreiro. While the Camino is something you'll never forget I think it really starts when you decide you want to go. Ponferrada will add texture to your journey and is some of the most outstanding scenery there is.
Other routes a also something to consider, but I don’t have any experience with them.
I'd suggest starting from St Jean Pied de Port or Roncesvalles and walking to Burgos. That's about 12 days of walking through what I would personally consider to be some of the nicest scenery I've ever seen.
Should you wish to return to the Camino, that'd give you a good place to pick up from.
You could see 2 countries in 2 weeks if you start from Porto, it could be done easily in 2 weeks, taking a day or two to explore the amazing city of Porto before the start, then 9-10 days to walk to Santiago, and then a couple of days after the Camino to stay in Santiago or head to Fisterra/Muxia. Portuguese Camino is easy for your feet, infrastructure is great, warm weather in April, very popular but not so busy as other routes.