As for buying a new one (as in a new Macbook), the question is whether or not the current Mac does what you want.
If it does what you need, then it is good the way it is.
If it is close, but could be a little faster, consider adding RAM.
If it is a little worse than that, consider replacing the internal hard disk wtih a solid state disk (SSD). But we are starting to talk some serious money now, so think long and hard if you need to go down this route.
If it is just not cutting it, then buy a new Mac.
I generally keep a Mac for 4-5 years, then the need for a new toy gets too strong. But I could most likely streatch it out a little longer if I had to. But even then, I may retires a Mac to a secondary life as a server. Or in the case of my last laptop, my Wife uses it as a 2nd systen not going though the VPN into work, when she has to access some web sites that do not really like work proxy servers (video and web conferences with companines outside of work).