How many days would be good for Osaka?

I've been to Tokyo many times, and now would like to visit Osaka. Only time I've been to Osaka was on a tour of Japan, thus, was in Osaka for only half a day or so.

About how many days should I dedicate to Osaka? Thanks in advance!

Hard to say ... depends mainly on how much of the city you'd like to see and whether you plan to make any side trips in the surrounding region.

2-3 days might be enough to cover the basics. Just as an example, I stayed a few nights in Ōsaka as part of a longer trip across Japan and was able to see the following in less than 2 days ("less" because I spent time outside the city on both dates):

https://withinstrikingdistance.wordpress.com/2014/07/23/field-report-o...
https://withinstrikingdistance.wordpress.com/2014/08/25/field-report-o...

Certain attractions could add a lot to the time required. A trip to Universal Studios, for example, might occupy an entire day.

Have a look at the Ōsaka page (including the "special interest guides" section lower down)...

http://www.japan-guide.com/e/e2157.html

...pick out the places you'd like to see and things you'd like to do. Based on that list, the community might be able to offer more detailed advice on how long you'll need.

Cheers.

If you go to Osaka, why not also go to Kyoto and either Nara or Kobe. These 3 towns are close to Osaka.

I am not the biggest fan of Osaka. We stayed 3 days during our first holiday in Japan and that was really enough. Even now that I am living near Osaka, I only go there for concerts or other activities. Great for shopping, but imo not much else to do. Regarding sightseeing, there is not much, but if you like food and nightlife, Osaka would be your town.
I am more a fan of Kyoto with all of its history, so in my oppinion Kyoto is the better town to spend your time in. If you have more time, why not try to visit more of Kansai then just Osaka? Kyoto, Osaka, Kobe or Nara are really near to each other and convenient to get to, so if I would be you, I would consider this.

I spent two days in Osaka and it was plenty, including an afternoon trip to Universal Studios (tickets are cheaper after 3 pm). It was enough to see the castle, the aquarium, Dotonbori, the Museum of Housing and Living and Shitennoji Temple. I did

(I did)n't go inside the castle, though; there is a museum inside that you could spend a couple of hours in.