You should set up daily, monthly, semi-annually, and yearly PMs.
Daily would include items such as: oil pressure to bearings, inspect collector for brush arcing, generator pressure if you have any insulating medium, cooling system if its applicable

Montly would include items such as: testing for shaft voltage, H2 purity and dew points, test field ground detectors, etc.

Semi-annual would include: megger field, clean collector, etc.

Yearly or longer PMs should include: Minor and Major insepctions, hI-pot, insulation leakage testing, check bushings and PT/CT, etc.