Personas are useful tools for understanding your audience.
There are many ways to build them, ranging from quick to complex. I once turned a set of personas out in about an hour that served the company well for several years. We didn't need a lot of detail - just a way to characterize the audience. At the other extreme, I built a set of personas for a financial institution that took several months. We telephone-interviewed about 100 customers, in depth and created detailed personas that described the customers. These also worked really well for the client.
How much work should you put into personas? I think -- just enough to do the job,
What you want is generally 4-6 representative personas that capture the essence of the audience you're developing for, You want to understand their mental models, their preferences and their values. When you know what's important to them you can design for them.
Here's an example of a brief persona. It's not extensive but you can get a pretty good idea of what's important to this couple:
Often, though, you want a lot more detail. Here's an example of a longer, more complex persona that I developed. In fact it's so long that I'm just posting a fragment here, But you can get the entire document by clicking on the button below the picture,
Personas are useful. But remember they are just another tool. There's nothing magic about them. If you get it right you can make good design decisions. You don't have to get 100% coverage with personas. Keep it to a manageable number and you'll cover most of your audience. Good design works for everyone so even the outliers are likely to be pleased,