"30 years worth" or "30 years' worth"?

[btz]

I'd love to move past this sentence, but I can't seem to figure it out.

"...30 years worth of company documents..."

or

"...30 years' worth of company documents..."

?

The "worth" is throwing me off and keeping me from going with the apostrophized version.

Any guidance leading to the alleviation of my mental paralysis would be incredibly appreciated.

[dfisher]

Traditionally it is with the apostrophe.
Doug

[btz]

Thanks, Doug!

[Neil Holdway]

I agree with apostrophe. I have a harder time with "$30 million worth of paintings." If you used the word "dollars," you'd have an apostrophe on it, too. Sometimes I rewrite to say "$30 million in paintings."