It depends. There are a number of factors that might contribute to how long it takes to feel better again. The most common include:
*The quality of your relationship with
*The amount of personal or vicarious trauma associated with the loss.
*The extent of unresolved issues in your relationship with the person who died.
*Your role in dealing with the aftermath of the death.
*Your willingness to address your grief rather than avoid it.
Bear in mind that bereavement is a process. Acute grief may last quite a while, and that's normal. More often than not, those who try to force themselves to "get over it" quickly are unsuccessful. Try to avoid this unrealistic expectation by being as patient and gentle with yourself as possible.
You may start to feel better in three months, but don't be surprised if you're still miserable, at least some of the time, several months to several years after your loss. The average length of time it takes most people to consistently feel better is about a year. However, it's also common to feel better for a while and then take a turn for the worse. That can be triggered by events such as special holidays or occasions that have a particular association with the person you've lost, especially the anniversary of his or her death.
A relapse of acute grief can also occur somewhat out of the blue. For example, on any random day you may find out about someone who is battling the exact same cancer your loved one had -- and this might trigger your feelings of intense grief all over again.
Finally, some people never really feel better. As they attempt to adjust to life without the person they've lost, they find it virtually impossible to derive joy again. Sometimes this takes the form of clinical depression, which can be treated with medication or psychotherapy that may mitigate the intensity of grief symptoms. For some, the inability to reengage in life results in suicidal feelings or attempts. I mention these possibilities not to scare you but to underscore the value of getting help if you need it.
Many forms of help are available. Sharing your emotional pain will likely help you process it and also help you recover from the disabling parts of grieving. If you're fortunate enough to have supportive friends, family, or community to turn to, take advantage of that. If not, or if you'd prefer to speak with someone outside your circle of support or trust, consider pastoral or mental health counseling or a grief support group to help you work through the myriad feelings that you're experiencing.
For the type of person who just doesn't feel like talking about feelings at all, the two most reliable activities are vigorous exercise and creative expression. When spirits are low, it can be very hard to find the motivation to take on either of these activities, but the payoff is worth it. It's usually best to start with something that you've already done, so you're less likely to resist the activity. For example, if you're already a knitter, consider knitting a memorial scarf or hat. If you're a runner, you may want to commit to three runs per week. A knitter will have a harder time getting started with painting, and a runner will be less likely to launch a rowing campaign -- at least at the beginning.
However, you may also find that new activities beckon. Many grievers are drawn to gardening for the first time, for example. Notice which activities hold your interest even if you're experiencing poor attention span, a classic symptom of grief. It's entirely possible that these whispers of interest are pointing you toward activities that will help you heal.
The tricky task here is to engage actively in your recovery process while simultaneously trying not to force a speedy recuperation. There's no blueprint that fits every griever. Make yourself the architect of your new life and start drawing up plans -- even if you feel you haven't the energy to do so.