Without experience, how are people supposed to know where they should feel safe! Personally I take a more scientific approach...
The primary aim is to turn at a safe speed which isn't causing excessive vibration due to imbalance from the wood. If there is too much vibration, slow the lathe down.
Once you have taken that into account, a rough rule of thumb can be used to give the answer to how fast can be calculated by dividing the diameter of your turning blank in inches into 6000 (or into 15000 if working in centimeters). This figure is derived by starting with 26 feet/second as the "optimum" cutting speed for wood. This figure is put forward by some woodworking text books and wood cutting machines often cut at around this speed.
The figure of 6000 is arrived at by rearranging the formula for circumference and peripheral speed to give rpm as a function of diameter.
For example, for a 2" diameter blank, the rpm can be up to 3000rpm and for a 12" diameter blank, the speed would be 500rpm.
I have used this rough rule of thumb for many years and find it a simple way to estimate turning speed because all I have to remember is 6000.
I recall in one of David Ellsworth's books, he suggests dividing diameter into 6000 and 10000 to give a safe working range. There was no explanation where the numbers were derived from but they at least agreed with my rough guide at the lower end.
Remember not all types of wood are the same so as with any cutting process, you might find certain pieces of wood respond better to slightly slower / faster speeds.
It should also be remembered that any rotating object is subject to centrifrugal force and if you double the speed, you quadruple the centrifrugal force (it's an exponential relationship). Excessive speed can result in the blank tearing itself apart with potentially catastrophic results so always check your blank is sound and free of major cracks, bark inclusions etc. and wherever possible, don't stand in the line of fire when turning.
A final suggestion - if you are turning an non-circular blank (such as a square block you are roughing to a cylinder), turning too slow will lead to the tool bouncing and will make it more difficult to get a good cut. Speeding the lathe up (within the safe contraints) will reduce the bounce and will give a cleaner cut.