On vh I never got above reliable. The normalisation is really hard on you.
I have a nice game with about 10 allies atm so maybe I can manage it, but it's hard:

The normalisation for allies is 1/400 to 1.0 while there's a normalisation of 1/200 to 0.0 in general each turn and 1/800 to -1.0 for each war.
This translate to a change (if you don't do anything) of:

a=a*199/200 at FactionTurnStart

a=1/400 - a/400 for each ally

a=-1/800 + a/800 for each opponent

Let's calculate the ally formula:
Let a be your global standing at turn start and n the number of allies
Our function is: f: R->R: a |-> a*(1-1/400)+1/400
So f(a)=a*399/400 + 1/400
Plugging this in recursively, gives us f^2(a)=a*(399/400)^2+399/400^2+1/400

Doing that a bit more often, we arrive at:
a*(399/400)^n + sum k _from 0 to n-1_ (1/400*(399/400)^k)

The sum term is the same for each a and only depends on n, and is nearly linear for small n (it's about n*1/400 because 399/400 is very close to 1)

For those more deeply interested, here's a little table with the first 11 values:
Spoiler Alert, click show to read: 
n sum
1 2.5e-3
2 4.99e-3
3 7.48e-3
4 9.96e-3
5 1.243e-2
6 1.490e-2
7 1.737e-2
8 1.983e-2
9 2.223e-2
10 2.472e-2
11 2.716e-2


The rest term obviously depends on a, so if we assume a start of turn value of 0, a number of allies of 10, with no enemies, we arrive at a rough turn gain of a bit less than 1/40, or 0.0245
The next turn, it'll be less; about 0.0240 resulting in a total of 0.0486
If I continue like this, to get to 0.25 which is the very reliable threshold if I'm not mistaken, should take about 12 turns.
However, if I break an alliance, have enemies or something it'll probably be far longer.

By the way, theoretically I should be able to get even to Very Trustworthy which triggers at 0.6, but it'd take 43 turns without enemies or any affronts.