The "typical" amount of time to walk from Porto to Santiago is about 10 days, plus or minus. You can take more or less time. Once you arrive across the bridge in Tui from Valenca, the flow of pilgrims increases. So too does the availability of places to stay.
In 2015, I walked five days into Valenca / Tui and another five days into Santiago from Tui. But, there were some longish days for me.
Leaving Porto, on the Coastal route, I recommend using the Porto Metro to skip ahead on the first day to Matsohinos, to avoid walking on sidewalks in a city. The next to last Metro stop lets you off on the Porto-side of a bridge across the river. There is a container shipping port on the other side of the bridge. About 15-minutes form the bridge on the north side, you walk clear of the container port and swing along the ocean proper.
Were I to do it again, I think 12 days might be about right (6 + 6).
I hope this helps.