How much can I enlarge an image for best quality on a large scale banner?

Where is this banner going to be installed? Further, from where will it be viewed? I ask these two questions because there really is no one-setting-fits-all standard for resolution of images printed for banners, signs, etc. It really depends on intended viewing distance.

150ppi at actual size for images is good for large format digital printed banners that will be viewed up close. One example would be a banner installed indoors at a trade show display or mounted on a wall inside a store. Adding to that, any lettering, logos, graphics with hard edges should be vector-based so they print as sharp as possible. JPEG is a lousy file format for logos, but so many people trade and move logos around in that format all the time. It's ridiculous.

Now if something is going to be viewed at farther distances the 150ppi at actual size standard is disc-wasting overkill. We typically produce imagery for vehicle wraps and outdoor banners at 72ppi or 75ppi. We'll still keep logos, lettering and other elements like that in vector form regardless, in part because they reveal a visible pixel grid much easier than photo imagery. Vector based elements are also far more file size efficient; they're easier to handle and consume less disc space.

Something really huge, but viewed mainly from vehicles (such as a billboard) can have pixel-based imagery set down to 25ppi at actual size. Many billboards are usually designed in scale, the most common standard being 1" = 1'. 300ppi imagery in a 1" = 1' scale billboard layout will scale to 25ppi when printed at actual size.