I can use a good quality cartomizer for a week or more (for me that is 2 weeks, vaping that carto 1/2 the time, because I use 2 models at once). I vape (I think) maybe 3-4ml per day of thin (around 80PG/20VG) light-colored clear juice with no particle residue (cloudy or opaque juices often have particle residue that clogs up cartomizers). Thin clear light-colored juices help cartomizers not get clogged up.
Sweet juices (even light thin clear ones) still have a tendency to need a fresh cartomizer sooner. I think the sweetener may gunk up the carto a bit quicker. But the main reason is sweet juices won't taste sweet if they aren't in a clean carto or if the carto has been even slightly singed by dragging it too dry. Singe is bitter and that cancels sweet. With clear non-sweet juice I can often get twice as much use out of a carto as long as I keep it damp (and the non-sweet juice can even coexist w/ a bit of singe)
I keep my cartomizers damp all the time. I do not "reuse" cartomizers - I continue to use cartomizers. I use a drip tip instead of the cartomizer endcap so I can easier pop that off to look in and see if the top of the stuffing has gone white i.e dry-looking. I can also judge how low the carto is on juice by the weight of the carto alone (off battery, no drip tip). When I add juice I do it with carto off of the battery and I make sure the carto isn't leaking and doesn't look like it will leak (if it looks too wet I gently stick a soft bunch of tissue in, tilt the open end down and puff lightly into the threaded end to get excess juice out.
Alternately you can run the cartos down to almost dry and try to avoid singeing them, then do a big refill at your convenience with whatever filling method you want.
Re cleaning - I don't try to clean cartomizers. When I start a new cartomizer I want it to be clean and work like new. Re cleaning atomizers. I used to use atomizers but they seemed to degrade despite frequent cleaning, and atomizer cartridges generally need to be filled more often than cartomizer. I prefer cartomizers for vaping my all-day vapes. I use a drip tip and atomizer to try new juices and to vape rarely used juices at home.
I would use cartomizers on that Ego - I like these on my Ego-type devices and they also don't seem to hurt my slim 510 manuals (3.2v batteries) - they may be OK on Blu too: Smoktek 2.5ohm Single Coil
http://www.madvapes.com/smok-tech-510-2-5-ohm-cartomizers-single-coil.html
3.0 ohm Boge cartos will be fairly cool on an Ego
2.5 ohm Smok Tek are in between
2.0 ohm Boge cartos will be much warmer on an Ego (and use the charge up faster)
Notes re Blu
1. I don't know about now, but Blu previously used very thick juices (VG or high in VG).
2. Blu is not a 510 (and in fact will fry on a 510 charger - reverse polarity) BUT it can use 510 cartomizers that are suitable for slim batteries (i.e. 2.5 to 3 ohm single coil cartomizers)
3. I am guessing you have automatic batteries. Refilling cartomizers is more tricky with auto batteries because they can more easily be shorted/killed by a juice leak out the threaded end of a carto. So you need to be extra careful that a carto is NOT going to leak after you add juice to it. And ALWAYS take the carto off the battery when adding juice. Carry tissue with you too. And I recommend you get a spare battery if you will be refilling the Blu's cartos.