depends on how well you take care of him--- rhoms are noted for their slow growth but will grow pretty fast up to 6" or so... so with proper care you would see good growth right away. could just hold things off till you get that bigger tank?
the rhom will grow about 1/2 an inch to an inch a year so you have some time but personally 1 year is enough then u have to get a lot bigger tank a 55 will last 2-4 years but sooner or later the 125 gallon- 150 gallon isa must seein they get 10-14 inches long and need a big tank to live a good life
i would strongly suggest you go bigger
got my gold diamond rhom in a 35g at 2in.....
in about 8 months he was 5-6in and was very crowded
so as long as you upgrade in less than 7 or 8 months you should be fine
BTW do like 25g each week in water changes and he will explode from growth
also some grow quicker than 1in per year
mine is 8.5in now and i got him june 05