I think it was about 6 weeks in the end, but that was after a couple of weeks of high dose of ferrous sulphate, plus some lower doses, a bit of Feroglobin and all the meat/veg, so my iron had already come up a bit by the time I started it. Also this was long before I was pregnant so there wasn’t that pressure.
How low is your iron? Depends what your starting point is as to how long you’ll need, but I imagine if you’ve got a twin pregnancy and are coming off the back of HG then yours will be in your boots and they might not think you can afford to be on the milder treatment.
I’d go and speak to your GP ASAP if you can. It might be that reducing the dose of the ferrous sulphate will help. See what they say. It’s unusual though - iron supplements usually back you up horribly rather than giving you the squits!
If you can bear it, make sure you’re taking every opportunity to eat iron-rich foods as well. Obviously red meat, but also fortified cereals, any dark green/leafy green veg, pulses etc. Any vegetable sources, have them with a glass of orange juice to boost absorption, and take it easy with the dairy because calcium inhibits iron uptake.