In my opinion, as little as possible. My reasoning is that it doesn't really help to build attraction, and there's a huge risk of it killing attraction. Why take the risk? On the other hand, it's really, really hard to put this into practice. If you like the girl it's almost impossible to prevent emotion creeping in. But I think it would pay off in the long haul. I've had a number of issues with too much texting lately. Let me explain. Hope it helps.
Firstly, with my recent trip to Japan to do daygame, I ended up with a shitload of numbers in my phone of girls that I didn't have time to see. One of them I particularly liked, day before I left she texted me something like "do you have space?" and I misunderstood and made some dismissive joke about being a spaceman, her English was not too bad but I didn't expect her to use that word in the context of dating window. So I thought it was pretty much over. But she continued responding and I hit her up every few weeks, however I did not really make clear that I was coming back to Japan and was intending to see her at that time. I over-invested and tried to tease/flirt over text which didn't come off right. She's now stopped responding. Sigh. So, I learned a lot about texting game. But I'd say it's too risky to text a lot, unless your texting game is a lot better than mine. Far better would have been no contact at all, followed by 3 months later a "hey, I'm in Osaka now, let's meet up if you have space". In fact, all the girls followed a similar pattern -> hit them up every 2 weeks or so, they respond less and less enthusiastically, and stop.
Now another scenario which I think is probably more relevant to you, coming back from JP I called in to visit a girl I was long gaming over Skype and it went super well, at the end of the time she asked if she was my girlfriend and I said no, I can't do LDR. So we continued to text several times a week (this was actually a precedent that had been set before I laid her), and I went to see her a second time, and it got even more serious, and we continued to text every 2 days or more frequently. I have the feeling this might be why it's gone sour -- too much exposure to me, although there are some other issues too that I've posted in another thread. Now it's kinda dried up, she responds if I text but no more. I reckon that's what'll happen if you keep texting this girl -- she'll be enthusiastic at first, but gradually you'll become too available and a burden. Especially if you're going for a FWB frame, DO NOT TEXT, the only way to play it with FWB is basically a casual "hey, I'm in town, let's hook up".
Ray