You may apply to sponsor a common-law partner, of the opposite sex or the same sex. If so, you have to prove you have been living with your partner for at least 12 consecutive months in a relationship like a marriage.
That means living together for one year without any long periods where you did not see each other. Either partner may have left the home for work or business travel, family obligations, and so on. However, that separation must have been temporary and short.
A common-law relationship ends when at least one partner does not intend to continue it.