Your title question is how often you get paid, but your post is whether this is easy. Each job comes with a set of instructions. The instructions sometimes are awkward, but if you complete them and report them as instructed, you will get paid. Yes, there are a couple of companies that are not paying when and as contracted, but the vast majority pay on or before their stated pay cycle and don't mess around with doing 'deductions' because of often seemingly bogus 'reasons'.
As for when companies pay, some companies such as Market Force purple portal and Corporate Research International pay 2X per month. Generally their shops pay less for comparable work elsewhere in part because of that 'two week payment'. Other companies such as Market Force blue portal, Service Intelligence and others pay on X day of the month following the month in which you did the work. Still other companies indicate they pay 30 days or 6 weeks or 3 months or 4 months after you do the job. So you just need to watch what their 'pay cycle' is.
The most frequent reasons for non-payment (called shop rejection) are due to: shopping the wrong location, failing to get the required proof of visit, failing to do all the required observations, failing to get the shop reported on time.
I know of few shoppers who have not shopped the wrong location at some time or other in their shopping career. We are visiting locations of chain stores and when there are 4 of that chain in a 5 mile stretch of a particular road, it is easy enough to go to the wrong place. Shopping the wrong time frame or date can also be a problem, but happens less frequently than shopping the wrong location.
A proof of visit is most often a receipt for a purchase or a business card. It doesn't matter if the location failed to give you the receipt. It is the only proof you were there so you MUST ASK if they didn't voluntarily offer it. Otherwise expect not to get paid.
Failing to do all the required observations is a little more subtle and some companies will work with you on it while others just throw out the shop. Part of the reason they can sometimes work with you is that the particular data is not critical to the overall shop. But if you forgot to check the restroom as required, expect the shop to be rejected. Don't try to wing it because you forgot or you can dig yourself into a huge hole. Own the error and hope for the best.
All companies have requirements of when they must report findings to the client. I do shops that the commitment by the company is 24 hour turn around. So if I did the shop at 4PM today, the client must have it by 4PM tomorrow. In that 24 hour period I need to report the shop, an editor needs to edit it and get back to me if there are unresolved issues and I need to respond back to the editor. This particular company gives me 12 hours to get my report in, which means their editors have only 12 hours to get the job wrapped up and available to the client. Maybe they can still salvage the job if I take 18 hours to report it, but they reserve the right not to accept it. Sometimes the website is down, which is no deficiency on my part, so the pressure is on their editors to hustle once the website is back up. Other companies state a specific cutoff time, such as midnight the day of the shop or 10AM the following day. If you don't live/work in the Eastern Time Zone, be very careful because this will be midnight or 10AM THEIR time, which may be earlier than your time.
All this being said, 90% of the shops a beginner are likely to be awarded are easy. Overall companies are fair in working with shoppers and pay for work in a responsible and timely fashion. The exceptions you will hear a lot of screaming about on the forum so it does make sense to check the conversation threads about specific companies. One or two shoppers having an issue you can pretty readily ignore. Lots of shoppers having issues needs to be your red flag to start your shopping career elsewhere.