Is there a limit to the length of HTML attributes?

How long is too long for an attribute value in HTML? I'm using HTML5 style data attributes (data-foo="bar") in a new application, and in one place it would be really handy to store a fair whack of data (upwards of 100 characters). While I suspect that this amount is fine, it raises the question of how much is too much?

[tohokuaiki]

HTML5の属性地のName,Valueには制限が無いとするのが一般的な解釈である様子。

[tvsk]

タグや属性の長さの上限。HTML 4 の時は属性の最大長さは65535. HTML5では上限なし、であるべきとのこと