Got your training organised for the week ahead?; go online and book into two classes and commit to attending them both.
The law of diminishing returns; research generally concludes that the minimum effective dose of resistance based training (springs, dumbbells, bodyweight etc) is twice per week.
Now, as always, there are some caveats to this; for example, if you have many years experience with resistance training you are likely to be able to tolerate more volume than twice weekly. Equally, if you are brand new to resistance training then as little as one session a week can stimulate a positive strength response.
GENERALLY speaking twice a week will provide most people with enough stress to trigger a change in strength with enough recovery time to ensure that the stress doesn't become detrimental to results.
Book now and commit; do two next week.