Thanks for your question.
If she entered the UK on a visitor's visa as a US citizen then she would only be able to stay for 6 months from the date she entered. She would not be able to apply to stay permanently during this time, because it is a condition of the visitor's visa that they must leave before expiry and this includes not applying for a permanent stay.
UKBA official grant visitor's visas to US citizens on a discretionary basis, so if you were travelling as a family and your wife simply wished to visit then they may not give her entry to the country if they suspected that she was actually coming to stay permanently.
Therefore if you wish to all travel together to the UK (when you do) so that you can settle permanently without anyone having to leave the you MUST apply for a spouses visa (settlement) in the US. There is no way around it. If you can prove that you and your wife have cohabited for a period of 4 years immediately prior to making the application then the UKBA should give you Indefinite Leave to Remain upon determining the application for settlement.
How long it takes to determine your application will depend. If the application is excellently drafted with all necessary documentary evidence and it is clearly laid out then it can take only two weeks. If there are issue over your eligibility and the application is not well laid out then it can take two months to get a decision and sometimes more.
If you wish for me to provide you with detailed guidance on making an application for a spouse visa so that she can come here permanently please submit a further questions to the board requesting me via my profile and marking your questions "FAO Tom".
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