From leaving your car to arriving back at your car, the Haystack hike is about 20 miles, give or take. For this reason, I think it is only do-able if you camp 1-2 days.
From JBL, it was about 12.5 mile day-hike to haystack and back to JBL.
^It is more like 16-17 miles (rather than 20) from the Garden, a reasonable 1 day distance. Anyone in good shape should be able to bag it in a day. Most of the people I know who have hiked it did in a day (myself included).