How often do I need to submit my site to the major search engines? I keep seeing ads that promise to submit my website to thousands of search engines each month for around $20. Is it worth it?
I don’t think it’s worth, you just need to submit some famous search engine manually, that should be enough. 
There’s only one search engine that truly matters, and you only need to submit once, although if you’ve got a link to the site itself somewhere I’d go as far as to say that you don’t even need to submit any more.
I can’t remember the last time I submitted a site to Google. Even my tiny, one-page CV site was up on Google within a day.
Sometimes I can make my website up in Google within some mins. Just show your link in a website that Google bot visits frequently.
Is it worth it?
IMHO, NO! - save yourself $20.
If your website has regular additional content or updates, then you can create a sitemap for Googlebot to aid the discovery of your new or amended content - but, even this is not necessary if your website is easy to navigate.
Put simply - never.
I created a new site last week and submitted the sitemaps to Google Webmaster tools and it has already found 8 of the web pages.
Do the same via bing and yahoo and it’ll sort itself.
Basically, submitting to any search engine is non worthy and waste of time. If you going for a paid service then it will be the great foolish thing. By doing this there will be no improvement that is going to happen your website ranking.
You can submit your site only once, that too the homepage of your website will do.
As Madeira said, just keep updating your site with keyword rich content. You don’t need to submit your site to search engines periodically, you need to make your site search engine friendly periodically along with adding good content.
I have no wasted my time submitting to any search engine since at least last century…
If you are using wordpress, install “Google XML sitemap” plugin, it will submit the updated sitemap to major search engines(google,bing,yahoo etc) every time you publish new post.
Maybe “submit” in as “submit XML sitemap”, but otherwise no.
Depending on your niche, you might want to submit your website to site/article/rss directories as part of link building. but submitting a single URLs to major search engines seems to me be a complete waste
I usually let the search engines find my site rather than submit to them and then only the 3 main ones count the Big G,yahoo and bing but i certainly would not pay to have them submitted.
Save your 20$ , Don’t waste your time and Money on this foolish thing. You can only submit your site in Big G , Yahoo & Bing.
Only once you need to submit your website to the all major search engine.
The fastest way to get your site indexed in the major search engines is to get a few high-quality links to it from other sites that are already indexed.
No you don’t.
my concern is there a monthly limit on to how many search engines you must submit to?
You should do this once as your website starts working and then there is no need to do this again. You do this once to get your website indexed by search engines faster and then there is no need to do this again, search engines already got it.
And what about directories how often should i submit to them?
Directories are as good as useless, but if you’re intent on submitting to them again only the once should do it. No one wants to see duplicate submissions, and I can imagine admins would be annoyed enough by it to reject a site.