1) Many, many years. Some people take more than 12 years before they start exhibiting symptoms.
2) You do not "contract" AIDS. Your infection begins to develop into AIDS. AIDS is a condition that is caused by the HIV virus. It refers to the substantial destruction of the immune system and subsequent infections that are able to attack the body as a result of this. Semantics aside, it is possible someone does not know they are infected until they have developed into AIDS.
3) No it would not. You need to undergo an HIV test to catch the infection.
4) As looking4answers80 has pointed out, it's a current STD that puts you at risk. It provides the virus with easier routes to get into your body, and sometimes it may signal that your partner may have been co-infected with the virus along with an STD. It has been shown that the virus is present in much higher concentrations of a person's infectious if a person is co-infected with certain STDs.
The biological aspects of having current STDs increases the probability of HIV transmission. For example if one has herpes simplex 2 (HSV2) and HIV the inflammation cells that appear near herpes outbreaks are loaded with HIV. You can read Dr. HHH talk about this in his comments in the expert STD forum. If you are the HIV negative party then this puts you at considerably greater risk than if your partner did not have STDs. So having current STDs goes both ways i.e. if you have them or if your partner has them. Use latex condoms for ALL penetrative sex unless you and your partner are tested together and are proven negative. HIV is not a game.
1) Years. Not everyone shows ARS (acute retroviral syndrome) symptoms, so people can go years without knowing they are infected.
2) Yes, it has happened, especially back in the early days when we didn't know as much about how it worked. But even today, there are people who get admitted to the hospital with pneumocystis pneumonia and are shocked to find out they have full-blown AIDS.
3) No, you are only tested for HIV if you specifically request it, or if it's done as a routine part of pregnancy blood work. There are (in most states) all sorts of legal issues around HIV testing. Many states require that positive results be submitted to the state health department so that partners can be notified of their risk.
4) A previous STD, to my knowledge, does not put you at a greater risk. A current STD puts you at greater risk. That is, if you have blisters or open sores due to herpes, that's an easier way for the virus to get in. If you have gonorrhea or chlamydia and are positive, the puss in your penis will carry a higher concentration of the virus. Syphilis can also make you more susceptible.