I've no experience with Dexters but I'll tell you how I find things with my Jerseys
For the first few days, the cow is producing colostrum / very rich milk, which you may not wish to use for yourself (it won't hurt you but some people don't fancy it.) If you have other calvers due, or sheep lambing, you may wish to draw some off and freeze it for other newborns.
Keep an eye on her, if the udder seems red, hot, sore, or too turgid, take some milk off to relieve the pressure.
By about day 4, the milk usually looks 'normal' so then you can start taking some for yourself if you haven't already.
My strategy is to leave calfie(s) with her 100% of the time until there's not enough for me! My Jerseys are high yielders, so they get a set-on alongside their own calf, and I generally find that there's plenty for me too for at least a month, maybe longer, before I need to take her away from the calves to build some up for me.
My Jerseys get between 1/2 and 1 bucket of cake a day when in full production - if I need more than she's producing, I up the cake and/or up the protein in the cake. I don't like them having more than 1 bucket a day each, so when she's getting that much and there still isn't enough milk for me, then I start to take her away from the calves for a few hours before milking.
I'll have been caking her in the pen as well as when I milk her, so the calves will have been copying her and have got the taste of cake.
When I take her away from the calves, I put some cake out for the calves. They only mouth at it at first, but it distracts them from mum leaving, and as they do start to eat it then they look forward to mum being put out and their cake arriving.
If you take her away for only a few hours - maybe three - at first, then calfie gets used to her coming back and being with it for a while, and doesn't gorge itself on her return. Then, if you need her to be alone for longer to build up whatever litrage you want, gradually increase the time she's away until you are getting what you want.
By a month old, calfie should be fine without mum for 8 hours. By 6 weeks, 10 hours. By 8 weeks, 12 hours.
Always have fresh water, hay and/or eating-quality straw and, when mum is away, a bit of cake available.
I wean mine at 4 months - you can wean earlier but I find the calves thrive better if I keep them on for at least 14 weeks. I like them to be eating a couple of pounds of cake a day when I wean them, then I keep them on the cake - just a scoop or two a day in two feeds - to at least 9 months old.
As you can tell, I love my cows and calves and love talking about them, so please feel free to ask any other questions!