When does it become MORE effective to use SEM than SEO for online marketing?

This completely depends on where you want to fall in a customers purchase journey!

For the search query "do red shoes help me run better?" you need to optimize your SEO Campaign to rank your page organically within the top 5 results.

But for the search query "best price for red shoes" you are much better offer running a SEM campaign, since idealistically this is what your competition would be doing.

This doesnt mean placing more resources on SEM & ignoring but to know when to use what based on your customers purchase journey.

SEM is more effective way but you must have a good plan .

I think, we are bit confused with SEM and SEO.  SEM is not all about search advertising alone. I think, Search Engine Marketing is a process to promote a product or service in search engine results pages through search engine optimization and advertising.  

Well, it depends on the campaign and strategy you would like to follow. SEO is a long term strategy you can invest in SEO to get long term results with minium budget of maintaining rankings & inquiries. While in SEM you should have good budget to be on top in order to receive good number of inquiries.

SEO  focused on optimizing websites by creating links to other website pages across the Internet. The SEM analyzed and managed keywords as a way to drive traffic.

With SEO, your goal should be to get on the first page, SEM can send more targeted traffic to your website than just SEO. Despite the differences, the most effective strategy is to execute both SEO and SEM. 

Despite the differences, the most effective strategy is to execute both SEO and SEM in your online marketing.

Both SEO and SEM involve keyword searches by the Internet user. The key for you is to find those keywords that can maximize both your SEO and SEM results from both organic and paid searches.

Today, many companies integrate SEO and SEM strategies. That’s because SEO and SEM are complementary programs that, when managed together, can bring bigger results than when done separately.

Beacuse in order to increase relevant traffic to your website instantly & increase the transactions or leads or sign ups or actions.

SEO takes time to rank your website and you have to wait for the results but SEM is easy way to target your audience quicly by spending money on advertisment on Search Engines.