The rules don't seem to clarify how many times a day a magic user can prepare a spell to cast.
The rules do indeed not specify, so you have two approaches:
First, recognize that S&W is based off of Dungeons&Dragons, and that in that game, you can only prepare spells after sleeping. I think the logic is that you need your mind to be in a fresh state. It makes sense to use the same rule in S&W, and the Cleric class has a similar restriction.
Second, since the rules don't specify, you can decide as a Dungeon Master that your setting works in some other way.
I'll describe how I've done things in my own Swords&Wizardry campaign to help illustrate how an approach different to the traditional D&D method might work out in practice.
In my S&W game, I allow a wizard to prepare a spell any time they have "headroom" (pun intended) for more spell levels. They must still spend the time preparing the spell, and cannot hold more spells in their mind at once than their level permits. In practice, I've found that this ruling allows low level wizards to be considerably more useful in their role as specialists. We have often had games where the wizard prepares a defensive spell at the beginning of the day, but brings along his spellbook, and winds up preparing and casting a variety of other spells as the situation demands. For example, after finding a locked door, the party may set up camp outside the door while the wizard prepares a Knock spell. (At higher levels, the wizard would likely invest in some scrolls for special use spells like this.)
The limiting factor on this is how long it takes to prepare a spell - In my game, I require 1 turn to for the wizard to prepare his materials, and then 1 turn per spell level to prepare the spell. I tend to run an "old school" game, and each turn spent in a dangerous environment gets a Wandering Monsters roll, so there is risk in standing still for long periods of time.