When should one commence taking antiretrovirals the instance he or she is diagnosed HIV+?

When an HIV- woman has unprotected sex with an HIV+ man for two consecutive days, what are the chances that she will turn up HIV+ and when should she commence treatment?

One can never if the HIV was transmitted to the woman but it is a very exposure altogether.The chances of HIV transmission depend on the viral load in the HIV + preson's bodily fluids at the time of sex,so we cannot determine if transmission occoured.
The only way to determine this is to take a HIV test.
It is very important to note that both the HIV + man and HIV - woman should not have sex with anybody to stop further transmission of the virus to others.
HIV tests are of 2 types,
Screening tests: ELISA (Enzyme Linked Immunosorbent Assay - the most commonly followed procedure). Spot test or Rapid Test These may give false positive results and hence should be confirmed.
Confirmatory Tests: Western Blot, the most commonly used procedure. There are other tests that are done to detect the virus or its protein or genetic material. These are however, expensive, complicated or may take long and hence, are used only for research and academic purposes.
The screening tests are again of 2 types,
Antibody tests :Look for the antibodies in the blood.This test will provide correct results if taken 3-6 months after risky exposure.
Antigen Tests:Proviral HIV DNA PCR Test:Looks for the DNA of the virus in the blood and can be taken after 72 hours of exposure but it is advisable to take this test 2 weeks after the risky exposure.
Antiretrovirals should be taken only on the advice of medical professionals.
What u need to do now are:
1.Visit a Doctor.
2.Take a HIV test as advised by the Doctor.
3.Abstain from having sex with anybody.

Hoping that the woman is not HIV +...
Infection due to HIV in adults occurs through:
1. Unprotected sexual intercourse with an infected partner. Regular and consistent use of condoms during sexual intercourse means protected sex and minimizes the chances of getting HIV and other sexually transmitted infections.
2. Transfusion of infected blood or blood products.
3. Use of infected needles as in injecting drug use or in health care settings.
4. From mother to child.

The A, B, C of HIV prevention are:
A= Abstinence
B= Being faithful to one single partner and
C= Consistent and correct use of condoms.

they both have HIV, so why get a test?

negative becomes positive over time anyway doesnt it?

prevention is by far better and cheaper than cure, however these are two questions all different. If this woman has already slept with this woman for two days, she should try ARV drugs that reduce transmission immediately and treatment comes after her CD4 count falls below 250 (own opinion)

(Exposure Route / Estimated infections per 10,000 exposures to an infected source)

Blood Transfusion 9,000
Childbirth 2,500
Needle-sharing injection drug use- 67
Receptive anal intercourse* 50
Percutaneous needle stick 30
Receptive penile-vaginal intercourse* 10
Insertive anal intercourse* 6.5
Insertive penile-vaginal intercourse* 5
Receptive fellatio* 1
Insertive fellatio* 0.5
* assuming no condom use
http://en.wikipedia.org/wiki/HIV

They should be tested about 6 months after they think they may have be infected. HIV doesn't test positive till about 6-12 months after being infected. If she thinks she may have contracted HIV she needs to be taking the medication immediately.

they should not have sex as it is not 'set in stone' when this could happen

some couples can catch this earlier/later it depends on many factors including how long the person has had it/age/ ect

If and HIV- person finds oout they have had sex with an HIV+ person, without a condom, they should IMMEDIATELY be seen by a doctor, who if they are up to date on the current protocols, will IMMEDIATELY start high dose multiple anti-retroviral treament. There is increasing evidence that this can prevent someone from getting infected if the anti retrovirals are started immdiately (preferably within 24 hours). Please see a doctor who specializes in HIV treatment immediately.

After 2 to 4 weeks, they can have their blood checked for virus using a PCR-DNA HIV test, which can detect the virus long before you have developed an antibody response (and thereby avoid having to wait up to 6 months).

I'm hoping this is just a rhetorical question... Although the percentages are relatively low on a person contracting HIV on first contact from penatrative sex (because the disease has such a low immunity).... there is always a chance that any form of shared body fluid can infect an individual immediately.
My advice is, that if you have been in this kind of relationship then to contact a GUM clinic ASAP, if for nothing else than to rule it out.
I am not judging you in any form, I just want you to be safe and well
Please take care.

Please see the webpage for more details on HIV infection. Please consult your doctor.