How much time does it take to earn something as a beginner in this field?

Define "this field"

For a simple answer, just keep uploading models and eventually you'll start to make sales. Focus on making high quality items, preferably in a particular niche, and sales will come in.

For a longer answer, how well you do on this site is dependent on a couple things:

Quality: Single most important aspect of your model. You can upload thousands of models, and you may even sell a few, but if the quality of your work isn't good, you will waste your time.

Use/Demand: Your models need to be desirable and useful. Most people aren't buying 3D models because they can't model themselves. Consider how useful your model really is. In my opinion, one of the major things people are buying for is to save themselves time. So usefulness could be looked as as "How much time does your model save someone?"

Price: How much money you make on this site is directly tied to how much you sell your items for. It's tempting to sell really low with the mindset "I'll make lots of sales if I sell low priced!" but this thinking only hurts you and other sellers. If you make high quality models, people are willing to pay higher prices.

Quantity: If you want to make lots of sales, you need to have lots of items to sell! But keep Quality, Demand, and Price in mind. Some of the highest sellers on the site often have the highest amount of items available. However, you'd be surprised by the fact that some sellers have less than 100 items and still manage to be top sellers. This is because they sell something highly desirable or with high quality.

I haven't had as much luck selling as I'd like, but I have learned some things in the process, so here's some tips.

1. Focus on a niche, but don't hold yourself to it too rigidly. If you find that your niche isn't selling, divert your efforts to a different category.
2. Sell high value, high quality items. There's no model on this site selling more than a few times a month, so expecting to make any decent amount off of $5 models is highly unrealistic.
3. Package and upsell your models. CGTrader has a great collections feature that lets you combine models into one product. Buyers seem to like buying in bulk, so it's super useful if you have a lot of items.

What Empire said ^

Niche is the key I think. Find an area that you're good at and beat it to death with a wet fish.