Hi.
I'm not a medic (I'm a mechanical engineer) but had low iron levels = low haemoglobin and very low Ferritin last year. (Had gastro and colon oscopy to check for a cause; no fault found).
Ferritin level was 7 (vs normal range of 30+) and Haemoglobin 120 (normally 130-180). Normal cell count, just all the red cells smaller that usual due to lock Haemoglobin due to low iron.

I was on iron supplements as a result for about 3 months.

It took about 6 or 7 weeks to be near normal. The next blood tests I had, about 7 weeks after the 1st, Ferritin was up to 50 and Haemoglobin 140.
I felt 'back to normal by then.

Next test a further 7 weeks later, all increased further, all good.(not got the figures to hand).

Retested this March (pre covid... just in case it was going wrong again) - all fine.

My own n=1 experience was that when the levels were at their lowest, I could still pedal all day on a v steady ride - I got 80+ mile hilly rides in when like that. But could not go above about 60-70% 'normal' power, and if I tried to sprint, I'd get 10-25 second then be totally knackered and hanging over the bars blowing hard as I could !
My max heart rate dropped too - from a usual of around 180 max, it wouldn't go above 130 for a couple of weeks (I was utterly gasping for breath at 130).

It was an incremental, steady improvement over the weeks. No sudden big bang.
This links to my understanding that iron adsorption is poor in people, so after emptying the reservoir, it just takes time for the iron reserves and Haemoglobin to rebuild.

My worst point was at the end of July. I was able to get a 116 mile ride with 10,000 ft climbing in at the end of Sept in a miserable weather (defo slower than I should have been capable of, but still did it). Did a half marathon PB in early Nov (I'm slow, but still was a PB ! And waaay better than expected).

What I didnt have is any known internal bleeding, and no transfusion- so I don't have any experience for the recovery of that in itself - just the effects of low iron and Haemoglobin, along with some repeated chest infections.

Hope the recovery goes well.