Well, I've made it to 5bn. Granted, I was already at 4bn on Saturday, I appreciate that it would be a more daunting prospect if I was starting from scratch.
My advice to those who can't face doing that, would be to do something else for awhile that happens to pay reasonably well (even if it isn't in the same league as mining for credits/hour). Exploration, for instance: just make sure you can get to Powerplay rank 5 with LYR before you set out (if you do it by haulage, be prepared to pay 100m to fast-track 10,000t of PP cargo). That will triple your payout, my 2.3 billion from DW2 was a big help. Robigo passenger runs to Sothis are still worth around 80m/hr when allied, and you might like to check out stacking cargo/data missions between Ceos and Sothis: I was going for Fed rank, so only taking missions from the Fed faction and going for Rep rewards, but I still manages to accidentally earn about a billion doing that, so probably several billion if I'd taken the most lucrative payouts from all factions.
My advice to those who can't face doing that, would be to do something else for awhile that happens to pay reasonably well (even if it isn't in the same league as mining for credits/hour). Exploration, for instance: just make sure you can get to Powerplay rank 5 with LYR before you set out (if you do it by haulage, be prepared to pay 100m to fast-track 10,000t of PP cargo). That will triple your payout, my 2.3 billion from DW2 was a big help. Robigo passenger runs to Sothis are still worth around 80m/hr when allied, and you might like to check out stacking cargo/data missions between Ceos and Sothis: I was going for Fed rank, so only taking missions from the Fed faction and going for Rep rewards, but I still manages to accidentally earn about a billion doing that, so probably several billion if I'd taken the most lucrative payouts from all factions.