It's very doable in 17 days and as already said the average per day is at 27km. You can also shorten two days a bit. First after Leon if you choose the Camino via San Martin del Camino and before Sarria if you don't go via Samos.
Stages could be like:
- Fromista: 24,7km
- Carrion de los Condes: 18,8 (+10,9km to reach average)
- Moratinos: 29,7 (+8,4)
- El Burgo Ranero: 27,2 (+8,4)
- Arcahueja: 30,3 (+5,3)
- Vilar de Mazarife: 29,2 (+3,3)
- Astorga: 31,4 (-0,9)
- Foncebadon: 25,8 (+0,5)
- Ponferrada: 26,8 (+0,9)
- Pereje: 29,3 (-1,2)
- Linares: 26,0 (exactly in average 27km per day)
- Samos: 27,7 (-0,5)
- A Pena: 28,8 (-2,1, many possibilities to stop earlier on this stage)
- Portos: 27,4 (-2,3)
- Boente: 25,6 (-0,7)
- O Pedrouzo: 27,8 (-0,1)
- Santiago: 19,4 and there you are in 17 walking days.
Buen Camino!