I think that the key to building up your BC is to train it as you would any other muscle. For once we are actually talking about a muscle, so work on exercising your BC muscle to failure. I initially couldn’t hold mine for more than 1-2 seconds, but I worked on it on a daily basis, building up to 3 second squeezes, then 5 seconds, then upwards. Do as many as you can at the set length (starting short) until you can’t do any more. Then after a week, add a second or two, eventually building up the strength until you can hold it for as long as you want. I have been holding mine since I started typing this over a minute ago and I could probably keep it held for a couple more minutes before it becomes uncomfortable.
It is the same as all PE, keep working, focus on your training and the gains will come. The advantage of BC training is that you can do it anywhere and anytime and no one will know. Squeeze on the way to work, while sat at work, through boring meetings, while watching tv, any time you have free, you can squeeze and the strength will grow.
LB
edit: I’ve now been holding the squeeze for over 5 minutes, so it is possible to train your BC to be really strong.