Quoted from the SFA Handbook:
3.7.2 On accumulating 18 penalty points in a League competition, a player will
automatically incur a suspension for a number of matches, starting with one match
for his first 18 penalty points, and increasing by one match for each successive 18
points accumulation, within a two season period. In effect, a player's record over
the current season and the immediately preceding season will be taken into
account.
should add this too:
3.7.4 Any period of suspension incurred by a player as a result of his having accumulated
18 or more penalty points, shall commence on the 14th day following the date of
the offence which resulted in the incurring of the suspension.The suspension shall
apply until such time as the requisite number of matches have been completed by
his club in the competition in which the suspension was incurred, in accordance
with the provisions of Paragraph 3.8.
think that just about explains the suspension criteria