aha, the dreaded 6-second rule surfaces once again
Another of RHC's contributions to folk magic, superstition, rumor, dogma, and confusion!
First off, opinions vary whether the six seconds (some manuals apparently say 5 seconds) is a Minimum or a Maximum. Nextly, is it a ho-hum or critical?
My (former) R22 service center declared my R22 unairworthy on my only incautious flight into their facility when I admitted the rotor was not turning after (gasp!) 8 seconds. Time they were through with me, I had a bill for $5000 (while adjusting the clutch delay, they discovered the bearing was worn, while installing the new clutch motor and bearing they discovered galling on the engine-sheave-layshaft requiring new shaft and fan, etc etc).
Later (having become proficient at adjusting the clutch microswitch myself, in the field, a Criminal Scofflaw wrenching on an Aircraft without mechanic's license) I found that outside air temperature variations changed the clutch-engagement-time between 4 sec and 12 sec, so now I just try to keep the average, ISA, time to about 6 sec.
Why does RHC care? Very clearly, it's intended to guarantee the starter will not be employed to spin the rotor blades, as RHC points out that event might result in bending one or more driveshafts (starter, or clutch shaft to transmission) because the starter is capable of delivering so much torque. Thus it's a minimum time.
Another R22 service center IA tried to tell me the six seconds is a maximum time, to ensure that the drivebelts will not suffer undue wear while rubbing on the sheaves before engagement. And that I should never run the engine without engaging the clutch, lest I wear a thin spot in the belts. RHC, presented with this claim, informed me that belt wear was not an issue, the engine could be run with clutch disengaged for short periods (warmup for oil change, perhaps), and that six seconds (five?) is a nominal average only.
So it depends on which expert you talk to. But there seem to be no great quantities of accident reports blaming engage-time discrepancies for fatal accidents, so I suggest you need not worry excessively. Anything between, say, 4 and 10 seconds gets the job done, temperature is a significant factor, and only if the interval steadily increases over several starts would I begin to look for weakened belts stretching. As my belts APPEAR to be 13 years old with 1800 hrs TT, never having been replaced, I would say the belts are Quite Robust. Or the 20-30 A&P's who've worked on the aircraft are derelict in their duties, having made no logbook entries regarding belts.
And a note concerning belt preflight: having, from time to time, dropped wrenches and bolts and the like into the gap between belts and lower sheave, which if undetected before startup would possibly break the belt or seriously score the sheave, I now manually compress the belt around either side of the lower sheave, looking for lumps, before start after maintenance. Cheap insurance!