if you want %20 flavoring 3 mL is correct per 15 mL batch, most of the flavorings i use at around 5-15% but its hard to say how much YOU should use.
when i started mixing my own i made a lot of bad e-liquid some of it was so bad i had to toss it out.
when im messing with new flavors i use heat to speed up the steeping time.
i take a clean small glass, rinse it out and dry it very well. then pour my batch into the glass and let the glass sit in a sink or tub of hot water for a few hours stirring once every 30 minutes also every 30 minutes i drain all the hot water out and put new hot water in. this will give me a good idea of what the finished product will be like, but a few more days of steeping if a cool dark place will really do the trick.
do not use this method with the e-liquid in a plastic bottle. PET plastic (most common for plastic e-liquid bottles) will break down at or above 140 degrees F and leach toxic chemicals into the e-liquid this is why you must use a glass cup or bottle.
if you have a crock pot you wont need to use a sink or tub just dont let it get too hot.