Once you've been here for 6 months, you can start the application process for a UK license. If you want to start lessons earlier than that though to find out what is on the test (automatic or manual, which ever you decide), you don't have to wait for that 6 months to arrive.
Actually I don't believe it is the case that you have to wait 6 months. You should be able to apply for the provisional licence and take the tests at any time, as long as the UK is your main residence and you are on a visa that is valid for more than 6 months.
The new gov.uk website design is awful though, because they don't seem to have any of this information on there anymore.
Leaflet D100 however, which you can download (
http://www.dft.gov.uk/dvla/~/media/pdf/leaflets/D100.ashx) or pick up at a post office says the following:
You can only get a GB driving licence if you are normally resident in Great Britain
To be normally resident you must usually live in the United Kingdom for 185 days in each calendar year.
Applicants who are not UK citizens or nationals of another EU or EEA country will not be considered normally resident if they:
• do not have leave to remain in the UK, or
• are in the country on a temporary basis without leave to remain either while awaiting a decision to stay in the UK or following a decision refusing such an application.
Now, notice that it does not say you
must have lived in the UK for 185 days, it just says that you must be
normally resident in the UK for 185 days in each calendar year, and it also infers that you will be considered normally resident if you have 'leave to remain' in the UK.
Now, a spousal visa, while technically it is 'leave to enter', it is exactly the same visa as 'leave to remain', and it is valid for more than 6 months - the only difference being that the spousal visa is issued outside the UK, while FLR(M) (leave to remain) is issued inside the UK (an extension to a spousal visa is also called leave to remain because it is issued in the UK).
Therefore, as long as the UK is now your main residence and you have a visa that allows you to live in the UK for 185 days or more each year, you should be able to apply for a driving licence at any time.