What is an adjective for "requires a lot of work"?

Consider challenging

Calling for full use of one's abilities or resources in a difficult but stimulating effort [American Heritage Dictionary]

Alternatives are burdensome, onerous, and arduous, but all of those have somewhat negative tones.

laborsome

1.) Made with, or requiring, great labor, pains, or diligence.

toilsome

"Characterized by or involving toil; laborious or fatiguing."

exhaustive

2.) tending to exhaust or drain, as resources or strength: a protracted, exhaustive siege of illness.

strenuous

1.) characterized by vigorous exertion, as action, efforts, life, etc.: a strenuous afternoon of hunting.

2.) demanding or requiring vigorous exertion; laborious: To think deeply is a strenuous task.

3.) vigorous, energetic, or zealously active: a strenuous person; a strenuous intellect.

Also:

• labor intensive (already mentioned below)
• a chore
• requires perseverance
• requires persistence
• requires dedication
• requires exertion

If you want a single word for "a lot of work", consider "onerous" or "laborious", though I'm not sure "a lot of work" captures all the relevant obstacles to starting a business.

Try this one: labor-intensive.