45mg nicotine content in vape juice would be extremely high. I have never seen, or heard of vape juice being this potent. I'm not saying it doesn't exist, I'm just saying it's highly unlikely it is that high. If it is, use caution when vaping. For a bit of context, when I started vaping, I used one of the highest nicotine contents available in the store I went to; 18mg. I now vape a 1.5mg and most of my time spent vaping has been at 3mg. Most vape juices (90%) in my experience is between 24mg and 1mg. I would even still call 24mg very, very high. The 30ml more than likely refers to the size of the bottle, containing 30ml of ejuice. Without more context or maybe a picture of the label, I can't give you much more than that.
I just did a bit of research and it seems that the strength of the liquid is indeed 45mg. Be very, very careful when vaping this liquid as it is incredibly high. Also, consider dropping down the level of the juice if it's too harsh. Out of curiosity, what mod (vaporizer) are you using? More powerful mods require weaker nicotine levels.