In general, unos/unas and algunos/algunas both mean "some," but they are used slightly differently.
Unos/unas are more often used with countable nouns and imply an approximate quantity. For example: "unas semanas" (some weeks) suggests a non-specific number of weeks.
Algunos/algunas can be used in similar contexts, but they are often used to express "some" in a more general or uncertain way, and they can be used with both countable and uncountable nouns.
In your example, "unas semanas" is correct because it's referring to a countable, but unspecified, number of weeks.
So, the difference is subtle, but unos/unas tends to be used more when you're talking about a quantity that can be counted, and algunos/algunas is a bit more flexible for a wider range of situations.