My first reef was a 38 gallon....I did upgrade the sump within a year. I probably wouldn't have upgraded unless I was moving. I went to a 55 gallon. That got moved to a house in 3 years, upgraded a year later to a used 65 since it was a reef ready tank and a good buy. Downgraded to a 25 gallon 4 years later, broke that and replaced with the current 29 gallon which I've had for about 10 years.
I don't really choose a time frame and haven't been without a tank running since April 2000...it may be a mess and only have softies for a time, but things have been doing much better the last few years.