If you're going to follow up a written proposal with a phone call it's usually best to do that within two to four weeks max. Beyond that it's a little awkward. Realistically if the potential client had any questions you know they'd call you. It's probably best to just leave this alone for now and assume that at least for the moment they won't be needing your services.

One of three things is going on:
1. They are or have been "fee shopping" other architects. In this case they may have found someone else, or they may have moved on to scenarios two or three:
2. If it's a small project, especially a residential project, they have decided to go forward with the project but without an architect. This might mean they're working with a builder to design the project, or they're designing it themselves, or their friend's nephew's architecture-student friend has stepped in, or some combination or variation on this.
3. They may have decided - due to financial concerns or any number other reasons - to put the project "on hold" for awhile.

Of these, scenario #3 is potentially the best for you. People often put these things on hold for months or even for several years, but they rarely forget all about the projects they really want to do. It's not unusual to meet with a potential client, not hear from them, assume that they've decided against using your services, and then get a call out of the blue two years down the road saying "we're ready to do it, let's get going!" (and the biggest problem you have then is that they often assume that your fees are still exactly the same....)

We keep all these "no go" potential clients on our holiday card list and say hi to them that way. Some years this has resulted in one or two of them giving us a call just after the new year, saying "let's go forward with the project."

Just to clarify, they did call after the proposal was received. My question is what now?

Leave it alone, unless you have other offers for work that if you accepted would require you to turn down these clients, at which point you could call them to let them know that you will need a decision because you have other work pending. But, think of it as a potential relationship - if they don't call, it's because they're not interested, a family emergency has come up, someone else has stepped into the picture... it's not worth your time pursuing if they aren't obviously interested.
And, even if after several phone calls you 'convinced' them to go with you or start the project, their lack of response could be a warning sign - imagine how it would be to work with them on the project if they continue to be unresponsive.

betadinesutures,

Come on now!! You know you need to pick up the phone and ask them if they are going to give you the work. Negotiate with them. If they think the fee is too high, how else can they compensate you. Do they own boats, restaurants, an airline, a casino, god only knows. In other words don't just let it go, after all you still have a potential client. Don't lower your fee, just persue your client and negotiate with them. And ofcorse, ask the question to which you need an answer; When can I start?

you mean close the sale!

tectonic,
F-in 'A' Bubba [F'A'B].

Yes! Close the sale. If you don't want it bad enough somebody else will. Many times the clients can be indifferent because of lack of knowledge about architecture. Good luck.

I dont know the whole story but their lack of profesionalism by not following up with you should be an indicator of their intrest. If you call them, you have bitten the hook. Look away from the light, dont go into the light....