This answer will be satisfying or unsatisfying depending on each reader's perspective.
Impulse
This answer to Ratio of low-thrust slow spiral to Hohmann transfer $\Delta V$? explains that the total impulse for an assymptotically low thrust spiral from one circular orbit to a higher one is higher than that needed for a Hohman two impulse transfer by a factor that ranges from unity up to $1+\sqrt{2}$ at infinity.
@MarkAdler's answer contains the solution for a bi-elliptic transfer as well but I'll leave that plot as an exercise for the reader.
So one criteria for high thrust might be how close to a theoretical Hohmann transfer the a given engine might be in terms of impulse.
Time
In Low-thrust spiraling to escape, is the flight path angle (gamma) at C3=0 always 39 degrees? I plot some slow spirals of a low impulse trajectory. Another criteria for high thrust might be how close to a theoretical Hohmann transfer the a given engine might be in terms of time.


...where $x$ is the ratio of the higher orbit radius to the lower orbit radius, assuming (without loss of generality) that the lower orbit radius is $1$ and $\mu$ is $1$.
import numpy as np
import matplotlib.pyplot as plt
# From @MarkAdler's answer https://space.stackexchange.com/a/34115/12102
def Hoh(x):
return np.sqrt(2.*x / (x+1.)) + np.sqrt(1./x) - np.sqrt(2./(x*(x+1.))) - 1.
def Low(x):
return 1. - np.sqrt(1./x)
x = np.logspace(0, 6, 601)[1:]
plt.figure()
plt.plot(x, Low(x) / Hoh(x))
plt.xscale('log')
plt.xlabel('x', fontsize=16)
plt.ylabel('"Low to Hoh" total impulse ratio', fontsize=16)
plt.show()