Do proper nouns always get the definite accusative suffix, since they always refer to specific things?
Yes, along with countries, cities, planets, days, months etc.
What about people's names?
How would you say "What time will you see Istanbul?"
Also, in what context would you use this sentence? Is this seeing as in visiting, or going on a trip?
Maybe it can be Saat kaçta...
Soon INN SHA ALLAH
Why not "to İstanbul"?
Why istanbulu not istanbul