The meta description, not being visible content for the visitor on your page, does not currently seem to be a significant factor in how high the url is ranking on major search engines -- it would be too easily manipulated if it were a major factor. (However, having a unique meta description on every url can be a significant factor in getting into the main index in the first place on Google, rather than just being filtered out. But that's another story.)

That said, there is no hard and fast rule about length. Often search engines will display your meta description as the "snippet" just below the link to your site on a results page. The amount of space they allot for this seems to change all the time, but the worst that happens is that your description gets truncated.

Currently Google seems to display right around the number of characters you mention -- 156. Yahoo seems to be choosing their own snippets rather than using your description in many cases - and though they tend towards short snippets, still they can extend to as high as 160 characters. And MSN is showing the most characters (190+) but the snippet seems to be a hybrid taken from the meta description and another bit taken from the page copy.

The engines experiment with their snippets all the time, trying to maximize satisfaction for their end users, so these numbers shift, and shift again.

I'd say 156 total characters is pretty spot on. Just make sure your important words are early in the tag so that it reads well even if it does get truncated.