How do Protestants who do not hold Communion/Eucharist every week interpret Acts 2:42?

The simple answers is that Acts is descriptive rather than prescriptive. Like all the narrative books it gives us wisdom and informs our decisions, but out relationship to a narrative text is different to our relationship to a text of commands (although those can be embedded within a narrative, as they are in the Pentateuch.)

Now there is one command that speaks of the frequency of communion, 1 Corinthians 11:25-26; you should have asked about that instead.

1 Cor 11:25-26 (ESV): In the same way also he took the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.” For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes.

One command directly from the mouth of Jesus, and one explanation from Paul. But it's far from clear that Jesus's words require this act to become a formal ritual for the church. As Acts records, sharing meals together, in remembrance of the death of Christ, was a common activity for the early Church. I think the common practice of giving thanks to God before a meal is a faithful way to obey Jesus's command.

There are many ways that churches have interpreted these verses. Many, taking note of "as often" have communion every week. Some Anglican churches actually have it every day. Some churches, rather than a token bite sized piece of bread and sip of a cup, have a full celebratory meal, like we imagine the early church doing. And in some Catholic churches the laity sometimes receive only the bread and not the wine, even though it was specifically about the wine which Jesus said "as often as you drink it."

But Paul gives us more than just this quote from Jesus, he gives us principles by which to decide how to implement this sacrament. The Corinthian church, by the time Paul was writing, had settled on a full celebratory banquet. The way they celebrated this banquet was deeply problematic however. Rather than sharing, some got drunk while others went hungry (1 Cor 11:21). Some may also have started eating before others even arrived, so Paul says to eat for the purpose of sating hunger at home, before you gather together (1 Cor 11:33-34). Paul says that we are to examine ourselves and others in the body to ensure we are worthy to eat this meal in remembrance of Jesus Christ.

I think this goal of serious remembrance of the sacrificial work of Jesus on the Cross can be achieved through different practices and frequencies of communion. For the churches which have it weekly (or daily) they want to ensure that the cross of Christ is never take for granted. By having communion this frequently they ensure that no matter what songs are sung, no matter what the topic of the sermon, the death of Christ is explicitly spoken of and deliberately remembered in every meeting.

For churches that have it less frequently, I have often heard the concern that weekly communion can become a motion you go through rather than a heartfelt act. I'm sure there have been church leaders who have felt at times that they've had to rush through communion because the preacher went overtime. While it is ultimately the individual church member's responsibility to take communion seriously rather than just as a habit to keep, the leaders of churches that have it monthly or quarterly hope this will help their congregants to not take it for granted but to be in the right mindset each time. When it is done monthly or quarterly, it can be something you set aside time for, you can include time for reflection in the service, you can choose particularly cross-centred hymns.

Personally I see merits in both weekly and monthly communion. I remember times when I was in a church that had it weekly that sometimes I treated it as just a motion to go through, and when I was in a church that had it monthly that if I missed the week it was on that I could almost forget it was something the church even did. I wouldn't want it to be less frequent than monthly however.