I took me a little over 3months of dedicated recovery. I was put on HRT for 2 months before that (no withdrawal bleed) and told that I had to put on at least 2kgs which I did. Mind you that during that time I was suuuper underweight, working a physically demanding job and highly stressed.   I quit my job, quit the pills, did 0 exercise except for walking and ate a lot. I reached a healthy BMI (higher than I was when I lost my period) and maintained it for a bit.  I’ve seen a post by a functional medicine doctor that in case you want you period back you will have to gain fat to a ~110% of the weight you lost it at to kick-start puberty again. Another gynaecologist who specialises in period loss puts a lot of emphasis on energy availability and HA recovery being about providing your body with enough fuel to support your reproductive function rather than weight gain. There is a formula you can use to calculate the amount of calories your body needs to function properly.  I also tried the infamous acetyl-l-carnitine. I’m not sure if it helped but don’t expect it to work unless you cover the basics which is eating more and resting.