Spiti needs good 10 - 12 days if you want to explore it and don't want to rush though places.
1. Delhi to Narkanda ( first day of journey makes it long )
I skipped Shimla considering you have upgraded from shimla tourist to Spiti Adventure traveler.
2. then Narkanda to Sangla , explore Sangla and chitkul ( in hurry then leave next day , however I would suggest two days )
3. Sangla to Rekong peo ( one day enough to witness the mighty kinnaur kailash and kalpa )
6. tabo to dhankar and then take de route to mud village ( one day )
Highest post office
Highest village
8. next day go to key monastery ( at least spend 4 hours to know the culture and heritage interact with monks and do wait for their tea and biscuits, their special tea helps you to acclimating in this region) then go to either kibber or gete village and explore the overnight stay . and chicham bridge
9. next day can visit rangrik village further go towards losar plains and towards chandratal
11. Manali to Delhi
These are the places you could explore in spiti circuit , but if you want to be tourist take selfie and rush , yes you can squeeze it to 8 days . that ways you will miss on places and I doubt many people once traveling through these treacherous roads will ever desire to return