Well there isn't a set standard length for a beardie's nails. Some people trim their beardie's nails, while others don't. It is really not neccessary to trim their nails unless they are getting so long that they are getting snagged on items in the enclosure, which could lead to nail breaking or even them ripping the nail out. The other reason people trim the beardie's nails is if they are scratching you when you are handling them.
If you do decide to trim the nail, you must be very careful to not cut into the quick. If you were to google "bearded dragon nail trimming".. there are actual videos out there that show you how to do it correctly.
I find for my beardie's that having a semi-rough tile floor(like slate) or a semi-rough slab of rock in their viv helps to keep the nails trimmed to a good length.
I hope this answers your question.