The average lifespan of the screen, which is a fiberglass mesh coated in nylon, is about 8-10 years if installed properly. The sun will deteriorate the nylon over time, leading to the screen becoming shiny and brittle. The average cost to replace the screen is about 1/3 of the original enclosure cost.
Typically, it is a cheaper square foot rate to have the whole enclosure rescreened at once, than it is to do it over time on a piece-by-piece basis due to material costs and time.
I screen every panel individually. Some installers will roll over the purlins in the roof to save themselves time, which leads to a screen panel that sags, and cuts at least 3 years of the product lifespan. No matter who you choose to complete your rescreen, insist that every panel is individually screened. It takes longer to install but it is worth it.
Most estimates that you receive will fall within a similar range. The brand of screen and material cost, size of the enclosure, difficulty level, equipment involved, and their availability will usually account for the differences in your estimates. Ask for a written estimate indicating what brand of mesh is going to be used and if they will be screening each panel individually.
Ask to see a copy of your installers insurance policy, don't just take their word for it. I have mine saved on my google drive, which can be accessed from my phone at a moments notice. If they can't provide it to you, protect yourself and hire someone else, even if it cost's a little more.