As HERC said...regular intervals!
I've used Triflow grease, Boca Bearings Heli Lightning Lube...and another synthetic "white" grease from my local bearing shop here in Indy. All with good results. Grease is like soda...lots of colors and flavors...you want one that is for the rpm range and type of bearing.
Intervals: Here's what I do...(any way?!):
Tail Grips and Tail output...every 4-6 gallons (the output are running close to 10K rpm...the T/R grips are flung at with great centrifugal force that literally works the grease out of them)
With all the exhaust oil that gets "worked in"...they need attention!
Main grips...8-12 gallons (or spindle/main shaft replacement due to dumb-thumbs).
All the little 4x8x3 in the head linkages...once or maybe twice a season (season = 150+ flights per airframe).
All the bearings in the blocks/frame...each new spindle/main shaft...if they weren't damaged in the event that cause you to pull these parts. IF you're getting 50+ flights between churning dirt sessions...maybe time to inspect and lube.
Keep in mind the main shaft is usually 1700-2100 rpm and though they carry the entire load of the heli...they're not under great centrifugal forces that fling the grease out of the bearing.
The clutch pinion support bearing is running at engine RPM...so 15k to 17K...give it your attention from time to time.
The Starter shaft bearings(3 of them) are spinning pretty quickly too...not exactly engine rpm but fast...check them when checking the clutch pinion.
Think that leaves the tail belt drive bearings...again, these dudes are spinning at the T/R rpm...so 9k-10k...give'em some love.
One point that I've learned this past season...if you've bent a spindle and/or Main shaft...there's a good chance you've damaged the bearings too. Bearings are silky smooth when new and when taken-care-of properly over their life-span. A bearing that you can "feel" bumps or rough spots as it spins...means the bearing has undergone an extreme loading or has ingested enough dirt to foul the balls and or races...replace it!!! Get enough rough bearings in an airframe spinning fast enough...and you may experience radio lock-outs/glitches...due to RFI (radio frequency interference) being generated by the "bumpy" metal on metal make/break contact going on inside a bad bearing.
Finally, when you're cleaning (alcohol, windex...what ever)...try not to get the bearings wet!
lotta ins, lotta outs