How long does it take HIV to show up positive in your blood?

It depends upon the type of test you are using:

To "Seroconvert" or develop antibodies to HIV, about 95% of people develop antibodies (Ab) within 3 weeks. Some people's immune system takes longer to develop HIV Ab and the window of time that an HIV screening test (antibody test) can take to become positive is 6 months from the time of possible exposure. This is true for blood or saliva tests that detect antibodies to HIV.

HIV DNA tests measure HIV particles and these tests can become positive more quickly than an HIV antibody test, but they are less accurate. In the case of acute HIV infection (very recent infection with or without symptoms) the HIV DNA test will frequently be positive before the HIV Ab test. HIV DNA tests are not normally used for HIV screening - they are only used if a person has possibly been exposed to HIV or is having symptoms of acute HIV.

Other tests, such as p24 antigen can be measured in acute or new HIV infection as well.

The "gold standard" test for screening and confirming HIV infection is the ELISA (Ab test) followed by a confirmatory Western Blot (Ab test) if the ELISA is positive.

up to seven years it can go undetected

Normally they like you to wait 3 months after the last time you had sex before they test you with any amount of accuracy.

even though HIV can lay dorment for a long period of time (up to 6 months) it will show up in a blood test in just a few hours as HIV spreads it becomes easier to detect. That is why it use to take so long for HIV test because they had to take a blood culture and let it grow to detect it, now however they have expanded through research and can find it much quicker.If you have a doubts about if you have contacted it or not it would be best for you to go to the health department and have yourself checked.