Dear Sarit
Fifteen days is more than enough to go from Porto to Santiago, if you are interested. It usually takes 10-12 days to do that. I did that trail last year and it was gorgeous.
Depending on how fast you walk, you can go start even before, in Coimbra, and still get to Santiago in 15 days, but it would require some long days.
In case you indeed want to stay only in Portugal, you can opt for start in Coimbra and stop at Valença, at the border, and you will have plenty of time to explore the cities. Or start in Tomar, but the trails before Porto are not that well marked, and the further you start (i.e. near Lisbon), the less structure it has.
Take a look at Gronze, with suggestion of staged all the way between Lisbon and Santiago:
https://www.gronze.com/camino-portugues