You can use them effectively as soon as you learn them, but you are more likely to become proficient within a week or two’s time. When you first start using mnemonics instead of your natural memory, you may notice that using mnemonics is a lot harder. Don’t worry about this; your brain will get used to memorizing things mnemonically.
When you are reasonably proficient, there are a few pitfalls you want to avoid. Firstly, don’t try to memorize everything just because you can, especially when you are just starting out. If you try doing this as a beginner, you’ll be bogged down by all the details, and you won’t have time to memorize everything that you need to know. When just starting out, try to memorize the most salient points first before trying to memorize the more obscure details of the material.
Secondly, don’t beat yourself up if you forget your images. At some point, there is a very good chance that you will forget the images associated with the information. This is normal. Oftentimes, you will still be able to remember the information, but not the images. If you can’t remember the information when prompted, you probably just formed an image too quickly or it wasn’t sticky enough for you to encode the information properly.
Finally, always continue to hone your memory skills and continue to read about people’s different approaches to mnemonics. You won’t be able to use mnemonics to its full potential if you just read from one source. When you read from multiple sources, you may find a technique in there that helps you memorize better than your current technique, or certain concepts may finally click into place.
As for techniques you should learn, I think that you should read Harry Lorayne’s and Jerry Lucas’s book on mnemonics. In my opinion, it is the bible of memory techniques, and it will give you a good foundation from which to build off of. You can easily find it for free if you look it up. I’d link it here, but I’m not sure if that’s allowed since Harry Lorayne and Jerry Lucas are still alive.