some people try putting a few drops of each into in a small glass of water to get an idea.
you can also taste the liquid (with your finger same as you might taste a taste a cake mix… but more sparingly ) soon as it’s mixed. worst that can happen is it tastes blegh, next worst is you can’t taste anything. yet. Neither method is a wholly reliable guide.
I think those flavours will go together just fine. Isn;t there some kind of insanely delicious indian sweet uses those two ? (my memory is fading due to longstanding food sensitivities). And usually, if flavours work together in cookery, they also work together in a vape; though there are certainly some flavours that are gorgeous to eat, but nasty to vape, which ruin that equation somewhat.
it probably won’t need a whole four weeks to steep. Some juices are ready to vape almost instantly (we call them shake and vape mixes ) Some take four weeks or more, depends on the individual flavours. And you get every shade in between .
I use rose extract myself, and find that steeps fast , That said , my brand (Medicine Flower) is almost certainly different from your brand, so we can’t make assumptions, but I’m hopeful that yours will prove much the same.
Cardamom is such a powerful flavour (in nature, at least) , do be sure to bear in bear in mind that the flavour might not come through properly til steeped.
I just typed “cardamon” into the search bar on the recipe side of this site. Looks like there’s lots of info on that FA cardamom, and a bit of info on some other brands. That should guide you in dilution %
http://e-liquid-recipes.com/flavors?q=cardamom
You can do the same with rose, ofc
Again, do bear in mind that different brands come in widely different concentrations, and even widely different constitutions (since many are chemical analogs , not natural flavours, for one thing) so the brand name is every bit as important as the flavour name when mixing it