30 minutes parking?

It is quite frequent. Offhand, the general rule appears to be that when a number of minutes (or in fact ANY unit of measurement) is being used as an adjective, "minute" is singular.

E.g.

24-hour fast;
the 4-minute mile;
a 15-minute break.

Or, with non-time-related units of measurement:

A 100-metre run;
A 20-mile hike;
A 5-ton truck.

30-minute parking says to me that parking [is allowed for a] 30-minute [period].

While you can run a four-minute mile then have a 15-minute break, you can also hand in your two weeks' notice and take 4 hours' rest. Early Modern English allowed for other variants on this, such as two hours' traffic of the stage.

In the examples you just gave, "weeks" and "hours" were functioning as nouns rather than adjectives.

The point was to show that constructions using plural forms also appear.

for those are confused the ' in the hours' show possession

As long as we use the hypen after the quantity, we need to use the unit of measurement (time,length,etc) in the singular form.

egs: 30-year-old man; 30-minute parking

Or, 30 years' of age; 30 minutes' parking in the plural form without the hyphen.

Nice comment Naeboo but please don't put apostrophes at the end of plural nouns.

Actually Naeboo's first example is incorrect, but his second example is correct.

the examples are all correct.please do some research before correcting. thank you.

i think the apostrophes at the end of plural nouns of measurement are quite acceptable too,gerry.

I disagree, naboo--the apostrophes have no place there. The apostrophes in such a spot would imply a possessive ownership, which is not the case. What are your thoughts on why to include them?

The hyphens are correct in my book.

the apostrophes being there do not mean there had to be some possessive connection to the object in discussion. they are mere illustration of the units of measurement.

that's what i have been taught to use anyways. and wld continue to do so. :)

maybe i'm wrong. please someone enlighten the legit reason why the apostrophes shldnt be there?

This explains some of the instances when apostrophes are used in relation to measurement:
http://www.ucalgary.ca/UofC/eduweb/grammar/course/punctuation/3_7.htm

This explicitly states it's for SOME cases only:
http://www.bcc.ctc.edu/writinglab/Apostrophe.html#T%20and%20M

This adds that minute or prime notation may sometimes require an apostrophe.
http://www.answers.com/topic/apostrophe-mark

Those are just the first rules I googled and found. Do they make a convincing case for you?

Are you from the U.S.? Maybe it's a regional preference baffling us?