I've applied to a couple advertising companies, and I haven't heard back from any of them yet. How long do these guys take to respond generally? Do they sometimes not respond at all?

Also, I have really lowpaying keywords and I'm getting less than 10 dollars a day with adsense even though I have more than 1.5 million pageviews a month. What am I doing wrong? Maybe I need a CPM solution instead of CPC?

It usually takes a couple of days to a week. Two at the most. All of the reputable ones always respond, whether you get accepted or not. They might be backed out now, so give them a chance - advertising is crazy during the holiday season.

As far as adsense concerns, I would investigate your CTR first. If you're not getting enough clicks, try repositioning your ad placements to see if you can raise that up. Even if you go with a CPM solution, no one is going to want to pay if your ad locations aren't performing.

What would a good CTR be?

It completely varies by your audience and demographics. Certain types of sites, such as forums, just naturally experience a lower CTR than other types of sites. It's the nature of the beast. Play around and experiment with difference ad sizes and positions, and when you find what works best for YOU, then stick with it. There isn't really a tried and true answer that works for everyone. A good CTR could be 0.2% for some and 2.0% for others.

if you are getting the click throughs on the ads then you might want to try revenue pilot since they have some good paying keywords upto $3 per click. Also you can give yahoo a go.

Good Luck