I thought it would be helpful to you to know how I test products. All products I test, I initially follow the instructions, and then after a while may find that a different way of using it suits me better and will continue to test it using various techniques.
Skincare: I usually aim to have nearly used up all of the product so that I will have already formed an opinion and I will have used it for a suitably long period of time, depending on the size of the item. I may use it for a length of time specified by the brand, or otherwise for at least 4-6weeks. I use the product as specified on the packaging and also in alternative ways if I feel it could be used in different ways.
Makeup: I tend to use makeup between 1-4 weeks before forming an opinion, around a week for eye makeup and upto 4weeks for foundation and primers. Other makeup types vary but I do test for at least a week depending on the product.
Haircare: I test haircare until I finish the shampoo/conditioner/treatment, this allows me to form an opinion on the product and it will be a true opinion and not first impressions.
All other Products: Everything else will generally be used for at least a week's worth of usage, depending on the product. Nail lacquers are worn and tested twice before reviewing to ensure that the miraculous flawless colour for a week wasn't a one time deal. For testing nail lacquers are worn for as long as possible until they chip or wear.
This is not set in stone, and I reserve the right to choose the period of time in which I test products. If it hasn't been tested for the minimum amount of times it will be classed as a first impressions post.