How long does it take to change a visitor visa into green card?

It is taking about 4-6 months to adjust status for a spouse of a US citizen.

If you and your wife have been residing abroad for at least the past six (6) months you should consider filing for her immigrant visa and I-551 "green card" via consular processing through the US Consular Section having jurisdiction over your current residence abroad. Because of the problem of "immigrant intent" based upon her marriage to a US citizen, she will not be allowed to enter in B-1/B-2 under the circumstances you describe when you opt to return to the US permanently. You should consult with an experience immigration attorney to guide you through the process of obtaining her lawful permanent resident (LPR) status.

Bringing your wife in on a tourist visa with the intention of having her stay permanently is immigration fraud. While you may be able to get her permanent residence with no trouble, if USCIS realizes she entered fraudulently there are VERY serious consequences. You should discuss your situation with an immigration attorney to figure out the best way to get your wife here, but most likely you'll need to file Form I-130 to petition for your wife while she is still out of the country. She can visit you on the tourist visa while the I-130 petition is processing since the fraud issue only arises if she enters on a temporary visa with the intention of staying permanently .