How long should I take lessons before deciding whether to continue?

IMHO, I think it is important to take several lessons since there are four stages of learning:

1 / People sometimes second guess their decision and doubt the need / urgency to start training

2 / They initially struggle & fear they won't be able to learn the new concepts, principles, and techniques 

3 / They start to feel, experience, and connect with the martial art

4 / They perform at a higher level on a more consistent basis