Playing any slot machine that has some form of bonus game can of course be fun, but only if that bonus game is being awarded to you regularly, and unless you play some certain types of slots, there is never any knowing just when a bonus game will be awarded to you.
Some slot game designers such as Real Time Gaming for example, have a range of slots on which you are going to be guaranteed of triggering a bonus game automatically if you do not trigger one in a set number of spins.
When playing their slot games which have a Feature Guarantee meter displayed at the top of the screen that meter will count down one spin at a time when you play off a base game spin, and when it reaches one spin remaining if that spin doesn’t see the required number of scatter symbols spinning in to trigger the main bonus game, then the slot will magically award the bonus game for you.
However, when playing a standard video slot for example that offers a free spins bonus game you could play off hundreds of spins before the bonus game eventually trigger, and in some cases it could take you thousands of spins before the required number of bonus game awarding symbols make an appearance.
In general, though, most slot machines you will come across online tend to award their respective free spins bonus games around the 150 base game spins mark.
Therefore what savvy slot players are going to do is to set their stake levels to ones that will allow them to at the very least play off 150 base game spins, as by doing so they will then have a very fair and reasonable chance of triggering the bonus game.