I have an ISFJ mom and an ISFJ sister. From personal observation I can say that they go to great lengths to make sure that your needs and desires are met. They are truly altruistic in love, willing to sacrifice their own comfort and desires to meet yours, even when the relationship is less than ideal. They will remain loyal even in the dark hours of a relationship and are willing to work hard and invest much to make it work. They will express a lot of love through practical, everyday things like doing dishes and the like (and I always feel like I don't value that enough).
I think ISFJs display a very loyal and steady love and possibly secretly feel like they love you more than you love them, but unless its way out of balance this won't really bother them much. They like to love and to express it and I'm not sure if there is a type more constant and sincere even when it's not a fairytale romance.
Just my observations. ISFJs, I hope you guys can relate.
Having been in a relationship with an ISFJ, and judging from that one experience, ISFJs are capable of a great amount of love, but they are, indeed, quieter about expressing it. So for example, my ISFJ's love language was acts of service (like he'd come over and without my asking fix my DVD player or the clock on my oven or something). I'm making him sound like a doormat, but ISFJs tend to be understated in how they go about things.
At the same time, they're not particularly assertive, and they might not actually speak up when they have an issue or problem (but that could have just been my ISFJ; he was an E type 9 as well). And my ISFJ had a martyr complex; like I self-sacrifice for you, what are you doing in return? That said, though, due to their Si they're pretty "stay the course," and with Fe supplementing it, they want to try to stay in a relationship and wok through things in not just their romantic relationships but friends and family as well. They're pretty sturdy, reliable, caring people to have relationships with, in my opinion.
Depending on the kind and extent of their romantic experience they may flip the switch from "not-yet-love" to "love" rather earlier than most people expect (and NFPs in particular in my experience) but that's the sort of thing I really can't imagine saying without it being 100% sincere. You can't say it without knowing it. And to me it's not really quantifiable, it's either "something-but-not-love" or "love" or "quite-a-mess-but-no-longer-love," so when you ask "how much" my first reaction is that we're simply speaking different and mutually incomprehensible languages.
So to really address your concern I think I have to speculate about why you're asking -- so if you're in a relationship with an ISFJ, then you might be able to address this by clarifying to yourself what exactly you need in a love relationship ("how much" and "in which ways" do you need to be loved? why did you post? be brutally honest with yourself; a healthy exercise in realism) and talking through with them what love means to the two of you. Just be open about that and if all goes well I'd expect that matters will eventually harmonize themselves between the two of you -- that their non-conceptualization of love will adapt to how much you feel you need to be loved, and / or vice versa, to the extent that the situation would support it.