see the title :)
Whatever is reasonable for the content that gets displayed...
Not PHP-specific, but I've read that if a page takes more than 4 seconds to load it's too long.
Possible citation: Akamai Technologies and it's also on Vincent Flander's Web Redesign Checklist:
Our home page — or any page — takes more than four seconds to load.
Totally, utterly subjective (thus, CW):
Anything between 0.0 and 0.4 seconds feels fine. Anything above it creates a notable delay.
On a public web site, when the average time to serve a page goes notably above 0.4 seconds, it is time to start thinking about caching.