There is a theoretical and a pragmatic answer to this question.
Without doing an oil analysis to determine the degradation of the oil in the unit, you have to use best practice. Oil does deteriorate over time, through oxidation, moisture contamination etc. So, to avoid problems on a unit that gets little use the oil should be changed every 6 months, thats what I do.
Alternatively you could adopt the pragmatic view, taken by a few of the respondents above and apart from checking level, just leave it! You may be lucky and never have a problem, who knows?!
My mechanical sensibility compels me to follow best practice, preventative maintenance, may cost me more in time and money but I have piece of mind.