Don't worry, if it is in the freezer it will not go bad for a long time.
First I disagree with Smurfe (Sorry Smurf)
When the rhubarb is taken out of the freezer you just slightly have to press
it and it will release juice immediately without effort.
Do not pulp-ferment the rhubarb, just ferment the juice.
Mind you rhubarb is very acid so I have written an entry in my weblog
about a way to remove the acid without using chalk (as is the usual method)
Look at :
http://wijnmaker.web-log.nl/wijn_weblog/2007/06/12/index.html
for the story.
Now rhubarb has a relatively high juice gain.
I had 712 grams rhubarb which gained me 550 mililiters (half a liter) juice.
So 1 kilo would gain about 750 ml juice.
Take this as a guideline and use :
http://web2.airmail.net/sgross/fermcalc/
If you want conversion to US measurements.
With this you can calculate the amount of rhubarb needed for the size of
carboys you have.
Luc