A general rule of thumb when it comes to retinol in your routine, if that for every decade old you are = how many nights per week you should be aiming to use your retinol serum. Example in your 40’s you should use Retinol 4 x per week.  

When introducing a retinol to your routine, it's always important to start slowly to avoid skin irritations such as dryness and flaking. My advice is to begin 2 nights per week, always follow with moisturiser 

 and gradually increase as tolerated over the next 4-6 weeks. This allows your skin to get used to the retinol and the increase in the skin cell turnover without the irritations you hear of. 

Retinols should be used at night as retinol can break down with exposure to the sun. They should also be used in conjunction with sunscreens as they can increase the sensitivity of your skin to UV rays.