Normally you only breed once a year. You can milk into the pregnancy for a couple months but she should be completely dried up at least 1 month prior to kidding. You want to make sure the kids get good colostrum.
If you have a goat that can go for longer lactations, then it could be a couple years before having to rebreed.
Alpines are seasonal breeders, the vast majority of them are. Meaning they will only come into estrus in the fall, normally august to feburary.
A doe should only be in milk for 10 months max.
You can still milk her while she is pregnant, but dry her up at least 1 month prior to kidding. Meaning slow her milk production by milking less for a while until she isnt milking hardly at all, then do not milk her anymore. She should not be milking at all 1 month prior to kidding again.
Normally it is not them that will slow down their own production if you are still milking , if they are from good milking lines they will milk for a long time. You have to be the one to slow her production.
Do you want kids every year, or do you want to grow your herd? If the kids are a pain and you just want the milk...you can try "milking thru"...so you wouldn't breed her again...you just continue milking and see how long she produces a good amount of milk. Do you have just 1 milker?