Sorry to hear you have lost your little mate Fluffy.It's always heartrenching to lose pets,however at least it was a natural death after a long and no doubt happy life in your loving care.
It would appear that there was no suffering as the cat had remained in the chair and probably simply went to sleep last night never to awaken again.I've seen this happen with people too,but usually most go through a period of confusion followed by semi-consciousness and later unconsciousness over a week or more.Sounds like your puss was one of the few who leaves this world in absolute peace after a life well lived.
Rigamortis generally begins within half an hour so I'd say he'd been gone a few hours or more.
