How long after turning vegetarian might anemia develop?

It shouldn't: You should not develop anemia as a vegetarian. Be certain to get the right amount of iron and b vitamins. If your foods do not contain the right amounts, you should take vitamin supplements.

You can prevent it: If you're a pure vegan- you are at risk to develop vit B12 deficiency.It'll take a few years to develop vit B12 deficiency anemia from being a pure vegan.You should take vit B12 supplement to make sure you are not deficient.If you still eat egg & milk- you can have some vit B12 from them-however may not be sufficient. Get supplement. Use B12 fortified food and look for cyanocobalamine on the label.

Hopefully it won't: Because your question tell me that you're smart enough to supplement. You're going to need iron and vitamin B12, and likely vitamin D, calcium, and a few others. Vegetarianism is the moral high ground and you actually can embrace it without ruining your health as so many of its adherents do. Get advice from a veg-friendly evidence-based physician or dietician.