RS Sailor Green
If Rainbow was the one keeping the Earth safe from outside threats, she probably hasn't been imprisoned for too terribly long. Otherwise the Earth would be overrun with whatever is out there. Also, if it's been so long, why is Black Rainbow only now trying to advance her plan?
I think the 20-100 year range would fit best. We can pick a date/year in that time frame to best fit our needs.
-nod nod- You're right. The imprisionment has to have been fairly recent unless we come up with an excuse for SBR to not be doing much.
Although SBR and SR and battled for a awhile before SBR had her imprissioned. The imprissionment then would have occured about 20 years ago. The "split" they under went could have occurred...well whenever the heck we want. As long as they both exist neither would die of old age.