I always have problems deciding if I must write a word in hiragana or in
kanji.

For example, should I write こんばんは or 今晩は ?

I know children would rather write in hiragana but what about for adults ?

Is there any rule (of thumb) I can follow to decide it or is it just left to
the writer's mood ?

Thanks in advance for your time,

Gilles
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word-choice
Is there a rule for deciding if we should write こんばんは or 今晩は?
While there are no rules set in stone, there are some generally followed
rules, especially when it comes to business communications. One of those rules
is that you generally don’t use the kanji form if it’s not that relevant to
how it is actually being used today, or using the kanji results in undue
gravitas.

For example, 「すみません」using kanji would be「済みません」, which is originally a
shortened way of saying “I can’t feel satisfied with my apology to you” as an
apology or “I feel bad because I can’t do enough to return what you’ve done
for me” as thanks. Nowadays that expression is used much more casually, and
using the kanji would not be entirely appropriate in most situations. Same
thing with 「ありがとうございます」=「有難う御座います」.

So it’s safe to say that any of the commonly used phrases to say hello or
farewell should be used in hiragana unless you are writing a rather formal
letter.
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> I know children would rather write in hiragana but what about for adults?

If 100% of your text consist of kana only without any kanji, that is 100%
weird. Kanjis are used not only to help identify what meaning for for sound,
but also kanjis work as visual dividers for text into small portions for
better eye-brain parsing of text, because there is no spaces in Japanese text.

> Is there any rule (of thumb) I can follow to decide it or is it just left to
> the writer's mood ?

There are cases, which are managed by some rule of thumb.

For example, here is a rule of thumb for two-rooted-verbs.

> 1) If the verb consists of two verbs and the first one is in -te form, the
> second verb is written in hiragana.  
>  For example,  
>  読んでみる [yon-de miru] not 読んで見る.
>
> 2) If the verb consists of two verbs and the first one is in -masu form, the
> second verb is written in kanji for its root and hiragana for its
> conjugation.  
>  For example,  
>  降り出す [fu-ri-da-su] not 降りだす and not ふりだす.

The real mastering of when to use kanji and when to use kana is a consequence
of feeling it. After a year of dealing with Japanese language on daily basis,
such feeling grows itself and there is no desire for any rules anymore.

> For example, should I write こんばんは or 今晩は ?

To make a quick check:  
go to Jisho.org.

For any word or phrase there is a mark "Usually written using kana alone".
Using Jisho.org to check for [kanji or kana] is the "rule of thumb" for
Japanese learners. But the final goal is to skip using Jisho.org and just feel
it.

To make deeper checking:  
go to tatoeba.org.

1) Type "今晩は" (use quotes to find exact match) to see examples of using with
kanji and what phrases are used to translate this form (in your case it is
[tonight], [this evening]).  
2) Type "こんばんは" (use quotes to find exact match) to see examples of using with
kana and what phrases are used to translate this form (in your case it is
[Good evening!]).  
3) Type "good evening", "tonight", "this evening" (use quotes to find exact
match) to see examples of translation: kana or kanji (in your case: check if
there is some usage of kana for [tonight], [this evening] and if there is some
usage of kanji for [Good evening]).

Result of this deep checking for your case is:  
1) They use kana to say [Good evening!]  
2) They use kanji to say [tonight] or [this evening].
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