Just under 40 weeks, 5.1 billion credits (9.4 billion Assets), a little mix of everything. Made 3 billion in bounty hunting/combat rest is trading/smuggling/CG/exploration.
1.5 Billion from Ancient Guardians mission (guide in my sig )
The rest from a mix of initial unbalanced stuff(like rares, 150M) , some exploit in Quince(not that much), lots of exploration and trading and buying credits online for real cash...
You could make yourself pretty rich (and Elite in exploration which would allow access Jameson memorial and all its modually goodness) by doing the "road to riches", you only need a small ship with decent scanners and big fuel scoop and maybe a 20ly jump range.
Got about 2.3 billion in cash. with total assets of around 3.5 billion.
Been playing since premium Beta and the first billion was from trade, the rest is from missions both cargo and passenger. I've seen a few posts saying credits were easier to make back in the day, but I'm not sure I agree. Whilst there certainly were some credit cash cows, there certainly weren't, for example, passenger transport missions paying 5-10 million for a couple of jumps.