I mean I ran a campaign that first was every 2~3 weeks, turning every month or worse when one player moved and we ended up having two Antwerp players, so in the end over 52 sessions, of which 32 were SR5, we averaged a bit over a month per session. I finished each run in 1 session, ended up giving an average of 6.5 karma per run based on the SR5 rewards. Adding nuyen->karma still kept them below 10 karma per month, which is about the point in SR6 where you'd be spending karma as fast as you're gaining it. So honestly, I don't think training times would have been a problem for us, nor were SR5 or are SR6 karma rewards.
But that's for a normal '1 run per month' campaign. If you're running anything else, then go ahead and do whatever you want, both training times and karma are explicitly guidelines. I ran a double-exp D&D 4e campaign from level 1 to level 10, because that was the speed I liked. I've played in short campaigns for Shadowrun where we spent everything as fast as we gained it, wasn't a problem there. If you want time to progress faster, go ahead and raise rewards in return. Want the players to go through tough runs? Go ahead and up the benefits. Playing a 30 Nights style campaign? Go ahead and toss out the training times. The system's not broken for the default style, but they acknowledge not everyone runs the same way, so feel free to modify using the baseline idea, so you have a rough idea to base it on rather than just random numbers.