I have attached an example of the pattern of loss of one particular gentleman, apologies for the quality. But you can see the frontal loss of this man at 27. The black hairline represents his "would be" hair transplant if he would have proceeded with a frontal restoration at that time. Look how his loss evolves up to the age of 40, and then over subsequent years his lateral humps(sides) and lower crown (which cant be seen) drop significantly. The last photo again highlights what would be left of his transplanted hair if he would have proceeded with that frontal restoration at 27 years.
Family history can be an indicator, but is also not definitive or obvious as males in your family will have varying degrees of loss.
Also health and lifestyle can have an impact. If you smoke, drink, active/inactive, diet, stress, anxiety, other medical issues or medication. Much of these factors become more of an issue in overall health as we age, which can have an influence.
When a patient reaches 40 years old for example, and as you explained, will not take hair loss preventative medication, his degree of loss and donor density will be influential in regards to what can be achieved and if his donor will be able to support any further loss in subsequent surgeries in the future.
This is why an in person consultation in this situation would be recommended before committing to surgery.