»Seid ihr am Wochenende zu Hause?« – »Ja, meistens.«
Does the above mean that they are at home most weekends, or that they are at home most of the time during the weekend?
There is room for interpretation, depending on the context:
In »correct« German the question
Seid ihr am Wochenende zu Hause? is referring to the next weekend (am is a contraction of an dem). So —assuming that the answering person hasn’t misunderstood the question— the answer should also refer to the next weekend, that is, next weekend they’ll be at home most of the time.
In this case, the answer
meistens is a bit colloquial (sloppy). A grammatically correct answer would be for example
Ja, die meiste Zeit.. I’m saying »colloquial« because it is not uncommon at all to use
meistens instead of the more accurate
die meiste Zeit.
If the interrogator wanted to know if they use to be at home on weekends he would have asked
Seid ihr an (den) Wochenenden zu Hause? In this case the answer would have definiteley the first meaning ›most of the weekends‹.
It’s not impossible to understand it the other way round (see @Ingmar’s answer): in this case you are assuming either that the answering person has misunderstood the question or that the question has been correctly interpreted by the answering person as a sloppy way to say ›Seid ihr an Wochenenden zu Hause?‹.
Indeed it could be interpreted like this but rather within a certain context. For example:
Im Juli und August, seid ihr am Wochenende zuhause? In this case it is rather probable that the question is meant (and understood) as ›Im Juli und August, seid ihr an den Wochenenden zu Hause?‹ and the answer
meistens would mean ›most of the weekends‹.
However, without any further context information the most plausible interpretation for the OP’s question is A): ›most of the time‹.
The former. Meistens means that they are usually (lit. most of the weekends) at home. Note that in this particular case the question usually refers to the next (coming) weekend: you wouldn't answer meistens then, which is a general reply. To indicate that you will be home most of the next weekend, use something like
Großteils. Den größten Teil.