Eligibility. You must have permission to stay ('leave to remain'). You must also have been in the UK legally for 10 years without gaps (known as your 'continuous residence'). This can include time on most immigration categories, or a combination of different immigration categories.
It roughly takes 5 years to get British citizenship, but you must be present in the UK with a valid visa (as a spouse, this is indefinite leave to remain).
Adults. An adult may be able to apply to naturalise as a British citizen if they have lived in the UK lawfully for five years and have been settled in the UK for at least one year. The spouse or civil partner of a British citizen may be able to apply after three years of residence if they are settled in the UK.
If you have indefinite leave to remain ( ILR ) You can usually apply for ILR after you've lived in the UK for 5 years. To apply for citizenship with ILR you must usually have lived in the UK for 12 months after getting it.
You'll usually get a decision within 6 months - some applications can take longer. If yours will take longer you'll be told before 6 months have passed. You'll be told if you need to provide more information to help with your application.
Marriage alone doesn't grant residency. Exceptions exist for those who are already British citizens or holding another visa, like a work visa.
UK immigration rules allow an automatic claim to British Citizenship if you have a grandparent (and, in rare cases, a great-grandparent) born in the UK. This is known as British Citizenship by descent, or more specifically, British Citizenship by double descent.
Yes you can. You will just need to wait to apply until you meet the continuous residency requirement of no more than x number of days abroad each year for so many years.
The spouse of a U.S. citizen who resides in the United States may be eligible for naturalization on the basis of his or her marriage.