When planning out your trade routes, things that should be taken into consideration
What is your ship and it's SCU (Standard Cargo Unit) Capacity?
How many credits you can afford to spend on each segment? (never spend more than 60%)
How long does the run take you?
How much overall profit are you making?
Are you flying through or landing in pvp zones?
How long will you be exposed to PVP danger while your ship is loaded with cargo?
Are you bringing escorts to mitigate your risk?
Do your escorts get shares of your profit?
Consider the risk of any run as compared to the profit / minute / individual
When calculating time spent, you have to consider the demand at a given location, this is currently bugged in the consoles so it's hard to tell, but you can't just dump 566SCUs of widow on a rest stop.
Trade terminal have buy/sell limits, the limits matter because it can take 2 minutes to sell 600 SCUs of widow at hex, or 2 minute to sell 1000 at Levski.
3.4.x Typical Progression for Trading:
Start with avenger, run widow from JT to Hex
At 30000 credits, switch to a Cutlass (if you can), run widow from JT to hex
At ~200,000 credits switch to your MAX (Assuming you have org coverage, otherwise stick with the cutlass to mitigate risk)
At about 500,000 credits running titanium between Tram&Meyers and PO in a caterpillar is a solid profit generator without too much risk
At ~1,000,000 credits, swap to Caterpillar and run med supplies out of PO into levski, returning with quartz
The fastest profit in game after 1.4MM credits is Load up the Caterpillar and haul meds from PO to Arc Corp 141 on Daymar, return with Agricium. Each direction nets ~125K, takes ~30 minutes.