temporary lighting

one of my two radion g3's shut down two days ago and had to be sent off for repair how long can a tank go with minimal lighting? my tank is a mixed 90 gallon and currently i have the single radion over the middle of the tank pretty high but obviously its not a long term solution. I may be just worrying too much but should i look for another light for temporary as the broken radion will not be back for a couple weeks thanks