Depends on the country you live in. In sweden it can take a few weeks, in most of the rest of the world, it takes a few hours to a few days.

Babies are born with sterile intestine but they quickly acquire some bacteria. The first ocasion being the passage through the birth canal, then feeding etc. So the mode of birth (natural vs C-section), the hygienic habits, the mode of nutrtion (breast or bottle) will definitely play a role in the acquisition speed of the intestinal flora. But in the end we need them in our gut to live a normal life.

Not only that. They also seem to be involved in the acquisition of immunity (maturation of the immune system) and even more importantly in the regulation of the mucus production in the gut. Axenic animals (with sterile intetsine) need special diets and usually have very pooor lives, but their situation is improved when they can acquire a microbial flora

Not really.
This is not something I have been studying in depth but the idea is that by challenging our immune system our internal flora is somehow improving the recognition between self and non-self. The consequence being that people which are not challenged enough tend to have a kind of hypersensitive immune system more prone to allergic reactions and auto-immune diseases. Last time I read about that, there were some fact to suggest this, but still more work was needed to prove it completely. Nevertheless this could be one explanation for the increase of allergies and other immune problems in the western countries (we are too clean).

Also intestinal flora is responsable for the formation of antibodies specific to blood types, which have epitopes very similar to the ones presented on the polisacharids of our red blood cells.

Yes if you are immuno compromised. In fact it is a problem for some cancer patients for example (I collaborated with someone working on this topic). Those are called opportunistic pathogens, and are not necessarily the bacteria the most suited for infections that starts this kind of infections.