There's no firm rule as to how much of a drug of dependence is too much to be considered intended for personal use. In Hanks v The Queen  VSCA 7 the Court of Appeal seemed comfortable [at 21] with the sentencing judge's rejecting a claim of personal use where a cannabis crop provided the appellant with enough to use 4 grams a day (the amount attested to by his doctor) for, 'about seven and a half years'.
When entering a plea of guilty for drug possession it's really important for counsel to establish before the plea starts whether the prosecution allege that the possession was related to trafficking or concede that it wasn't. It makes a significant difference to the penalties available.
Section 73(1)(b) of the Drugs Poisons and Controlled Substances Act 1986 casts an onus on the accused to satisfy the sentencing court on the balance of probabilities that the drugs were not possessed by the person for any purpose relating to trafficking. If satisfied, the maximum penalty for possession is a year of imprisonment. If the court is not satisfied the drugs weren't related to trafficking, the maximum penalty is imprisonment for 5 years.
A similar provision exists at s 73(1)(a) for small quantities of cannabis, and if the court is satisfied that the possession was not related to trafficking the maximum penalty drops to 5 penalty units, a bit less than $600.
The maximum penalty for an offence of cultivating cannabis under s 72B is 15 years. If the sentencing court is satisfied the cultivation wasn't related to trafficking, the maximum penalty is one year of imprisonment, a fine of 20 penalty units, or both.
Under s 76, many accused with no proven drug history are entitled to receive an adjourned undertaking if the sentencing court is satisfied that the offence was not connected to trafficking, and the court decides not to award a conviction. (Any history in the Childrens' Court is excluded as a conviction under s 76(3)). This applies to charges under ss 72B or 73 with possession of cannabis offence, or an offence of possession of a small quantity of any Schedule 11 drug.
Section 76 states that when deciding whether or not to impose an adjourned undertaking, the court shall having regard to the character and antecedents of the person and to all the circumstances and the public interest.
The Supreme Court considered the proper operation of this provision in Buckley v DPP  VSC (Unreported, 4/8/94, Ashley J) [at 4]:
That, then, called into operation the concluding portion of s 76(1). Obviously enough, the use of the word "shall" does not, where used in that provision, compel the Magistrates' Court to adjourn the hearing without conviction. There is a discretion to be exercised. On the other hand, the verbiage is also to the effect that a Magistrates' Court is to approach the matter by proceeding to adjournment without conviction, unless the alternative course of conviction is considered appropriate. That is, there is a certain skewing of the provision in favour of adjourment as distinct from conviction.
Such a reading is, I think, to be preferred, particularly when regard is had to sub-section (2). The Magistrates' Court is there specifically enjoined to state its reasons in the event it proceeds to conviction under sub-section (1). That appears to underline the relatively grave step of proceeding to conviction in a case ehere sub-section (1) has potential application to the benefit of an offender.
With such a big difference in potential outcomes, it's worth a considerable amount of effort trying to persuade a sentencing court that drugs were not related to trafficking. It's the statutory maxima for the offences, and not the limitations on sentencing in the Magistrates' Court, that define the seriousness of offences: R v Duncan  VSCA 137. While older authorities tend to emphasise that the statutory prima facie weight creates a presumption that must be rebutted, R v McCulloch  VSCA 151 suggests that the finding of a connection with trafficking must be supported by evidence.
The simplest way to satisfy the court that the possession was not related to trafficking is by gaining that concession from the prosecution. Under s 4 of the Evidence Act 2008, evidence is not required in a sentencing hearing unless the court deems it necessary or one of the parties requests the rules of evidence to apply.