Usually side effects show up right away and tend to lessen over time. Or that has been my experience with drugs anyways. Allergic reactions happen immediately, or very fast. Contact a doc or 911 right away of course if this happens! There are also the long-term side effects and they can take...well, a long time to show up. haha.
considering you're on the meth board I assume you're asking about meth side-effects. Apparently there are a lot (I usually don't read up on side effects considering I'm a bit of a hypocondriac, instead I let my mom learn all the crazy things that can go wrong.). I personally haven't really felt any beside nausea (I experienced it right away too). Though, that is to be expected since I'm on such a high dose of meth (25mg/week). Luckily, my GI gave me a crazy strong anti-nausea to counter balance.
You should always be getting blood tests as well, every 2-4 weeks to watch for those rare, albeit, nasty long term side effects.