Hi and welcome
Nails in guinea pigs (once they start growing out; the onse of which can vary) will usually have to be cut about every 4-6 weeks in piggies living on soft surfaces. They grow fastest in about 1 year olds and then very, very gradually slow down over the course of a life time but never stop growing. Depending on the genetic disposition, they will start to curl, twist or thicken as a piggy ages; the part that curls or twists is always dead so if you cut just a little less far, you are safe to not hit the quick in dark nails.
What you can do with youngsters is to gently fondle and manipulate the feet a little during handling time so they get used to it all. You can also use our piggy whispering tips (always a combination of gentle or stronger assertion of authority in combination with assurance that you still love them) to get better compliance; this is how my savviest boars have dealt with their rambunctious wives in a successful way - and this why piggy whispering is surprisingly effective in lots of contexts; including medication and other stressful situations.
Here is more information:
Guide to Cutting Guinea Pig Nails
Understanding Prey Animal Instincts, Guinea Pig Whispering and Cuddling Tips
Who is the Boss - Your Guinea Pig or You?
These guides are all part of our extensive practical information resource (one of the largest around), which you may want to bookmark, browse, read and re-read as needed. You will get different things out of some guides at different levels of experience.
Getting Started - New Owners' Most Helpful Guides
Since we can't repeat all the little practical details in every post (seeing that we are all doing this for free in our own free time), you will find that we link in the respective guides for more in-depth information and practical how-to tips.