If we install a underground conduit (say 3") between buildings (say 600 feet-ish or more) for fiber. For straight runs how far should handholes be spaced for pulling?

A few considerations:
Fiber mfg specs for pull tension?
Pulling equipment limitations?
Pull rope length?

600' straight may be 'one shot' pull.

EIA/TIA documents and specificaions likely have something to say about this. Other things to consider:

How many different cables with fiber?

Are they in innerduct? Pulling innerduct would limit distances. Some pre-terminated cables come in innerduct as part of the assembly.

Are they gopher rated cables being pulled in? Gopher rated means shielding against burrowing critters. Makes them harder to pull, but may increase allowed pulling tensions.

Is conduit sizing based on future pulls possible/likely?
Do the specs, manufacturer, or EIA/TIA allow soap? Soap is another name for a lubricant that eases pulling of cable.

For pulling refer also to BICSI ITSIM, Ch. 4, "Pulling Cable."

I am sorry that my memory is not as good as it used to be, years ago I helped with some revisions to the EIA/TIA documents, but I have forgotten more than I have ever learned.

Shnae

I can't even remember how to spell my name.

Shane