Classic analog chess clock for play with accuracy threshold

Hi all,

I've recently acquired a couple classic Jerger chess clocks and want to "certify" them for tournament like conditions for personal play. 

For accuracy's sake, how many hours should each side lapse before I declare they are accurately maintaining time?  Would 3 hours be sufficient for each side?  I know FIDE's typical time control is something like 1.5 hours for 1st 40 moves then 30 mins. sudden death (plus 30 seconds per move; not sure how I would implement this considering it's an analog clock.). I understand analog clocks are no longer allowed or the choice for use in most tournaments but like I said, this is for personal use, but I am concerned that my timers are accurate.  

Accounting for the longest time controls out there, would allowing for 3 hours lapse on each side be an adequate check in determining their accuracy for tournament like condition play?  How many hours would you let it lapse before you say this clock is accurate for chess play?  Thanks!